216 F.3d 611 (7th Cir. 2000)
UNITED STATES OF AMERICA,    Plaintiff-Appellee,v.KEVIN WILLIAMS, also known as  TWIN,    Defendant-Appellant.
No. 99-4265
In the  United States Court of Appeals  For the Seventh Circuit
Argued May 10, 2000Decided June 19, 2000

Appeal from the United States District Court   for the Central District of Illinois.  No. 98 CR 30080--Jeanne E. Scott, Judge.
Before EASTERBROOK, RIPPLE and ROVNER,  Circuit Judges.
ROVNER, Circuit Judge.


1
A jury convicted  Kevin Williams of conspiracy to  distribute cocaine base, in violation of  21 U.S.C. secs. 841(b)(1)(A) and 846.  Williams now challenges the sufficiency  of the evidence, the district court's  admission of evidence of other bad acts,  and his sentence. We affirm.

I.

2
Williams was charged with conspiring  with Eric Powell and others to distribute  cocaine and cocaine base. Powell was a  childhood friend of Williams who  testified under a grant of immunity as  part of a plea agreement with the  government. Powell testified that from  mid-1997 through October 1998, Williams  provided him with crack cocaine on a  regular basis. Approximately every other  day, Williams provided Powell with  quantities ranging from one-eighth of an  ounce to two full ounces of crack  cocaine. Sometimes Powell collected money  from the ultimate buyer up-front and paid  Williams for the drugs at the time Powell  received them. Other times, Williams  provided Powell with cocaine base with  the expectation that Powell would pay him  after he sold the drugs, a practice known  as "fronting." Powell and Williams often  communicated using cellular phones, and  many of their discussions were recorded  by the FBI under a court-authorized  wiretap. At trial, Powell and seven other  witnesses who knew Williams well  identified Williams' voice on the tapes.  The conversations were brief and in code,  as Powell explained at trial. Powell  called whenever he had a buyer and needed  to meet with Williams to replenish his  supply of crack cocaine. Using a language  that was all their own, Powell and  Williams discussed quantities and prices,  as well as times and places for the  transactions to take place.


3
Powell's girlfriend Yvonne Persley also  testified. She had been charged with  assisting Powell in his drug business,  and at the time of Williams' trial she  was awaiting sentencing for her role in  the scheme. She testified that Williams  was at her house with Powell several  times a week, that she heard Powell on  the telephone with Williams arranging to  receive a "package" for one of Powell's  drug customers, and that she twice saw  Williams in possession of wads of cash  the size of a tennis ball. Larry Austin,  who had been charged with conspiring with  Powell, testified under a grant of  immunity that he saw Williams sell crack.  David Jackson, another of Powell's co-  conspirators, testified that he purchased  crack directly from Williams during the  relevant time period. Finally, Powell's  brother, Deshawn Powell, testified that  he saw Williams sell crack to Powell in  the summer and early fall of 1998.


4
Before trial, the government filed  notice that it wished to introduce  evidence from five witnesses concerning  their observations of Williams possessing  and selling crack cocaine during the time  of the conspiracy, but not directly  relating to the conspiracy with Powell.  The court carefully considered the  government's proffer, applying the four-  part test set out by this Court in  analyzing the evidence. The district  court ruled that the prejudice of some of  the evidence outweighed its probative  value, and this evidence was excluded.  The district court ruled that other  evidence was relevant to method of  operation, motive to conspire and  availability of the drug to the  defendant, and that the prejudice did not  outweigh the probative value for this  evidence. The court offered to give  limiting instructions to the jury, an  offer that Williams accepted. At trial,  the court instructed the jury both at the  time the evidence was admitted and during  final instructions that this other bad  acts evidence was to be considered for a  limited purpose only, that it could not  be used as direct evidence of the crime  charged but rather could be used only to  show motive, opportunity and method of  operation. After the first two "bad acts"  witnesses testified, the court further  limited the government, believing that  additional bad acts testimony would be  cumulative. Although the court ruled that  the government could put on one more such  witness, the government declined to put  in further bad acts evidence. The jury  subsequently convicted Williams, and the  court sentenced him to 151 months of  incarceration.

II.

5
On appeal, Williams first asserts that  the evidence was insufficient to allow  any rational jury to find that he was  guilty of conspiring to distribute a  controlled substance. Williams  acknowledges the formidable hurdle he  must overcome to successfully challenge  the sufficiency of the evidence. See  United States v. Van Dreel, 155 F.3d 902,  906 (7th Cir. 1998).  Viewing the  evidence in the light most favorable to  the government, we reverse only if the  record contains no evidence from which  the jury could find guilt beyond a  reasonable doubt. Id. Williams claims the  government's case must fail because the  witnesses presented against him were not  credible, and there was no corroborating  evidence. The essence of Williams' claim  is that Powell lied to the government in  the process of reaching his plea  agreement, and as a result, the  government was forced to enter into a  second cooperation agreement with Powell.  Thus, by the government's own account,  Powell was a liar. Moreover, some of the  government's other witnesses contradicted  certain testimony by Powell. Williams  concludes that Powell was incredible as a  matter of law. Williams also complains  that the government failed to produce any  corroborating physical evidence, such as  the drugs Williams distributed. In these  circumstances, Williams contends, his  conviction cannot stand.


6
Credibility determinations are within  the province of the jury, and we reverse  such determinations on appeal only under  exceptional circumstances, such as "where  it was physically impossible for the  witness to observe that which he claims  occurred, or impossible under the laws of  nature for the occurrence to have taken  place at all." United States v. Ruiz, 178  F.3d 877, 880 (7th Cir. 1999), cert.  denied, 120 S. Ct. 229 (1999) (quoting  United States v. Hach, 162 F.3d 937, 942  n.1 (7th Cir. 1998), cert. denied, 119 S.  Ct. 1586 (1999)). Because Williams has  failed to allege any such special  circumstances here, his challenge to the  sufficiency of the evidence is without  merit. Ruiz, 178 F.3d at 880; Van Dreel,  155 F.3d at 906. We must leave open the  possibility that even a liar tells the  truth once in a while, and the jury is in  the best position to judge Powell's  credibility.


7
Williams next contends that the district  court abused its discretion in admitting  evidence of other bad acts committed by  him. Federal Rule of Evidence 404(b)  provides:


8
Evidence of other crimes, wrongs, or acts  is not admissible to prove the character  of a person in order to show action in  conformity therewith. It may, however, be  admissible for other purposes, such as  proof of motive, opportunity, intent,  preparation, plan, knowledge, identity,  or absence of mistake or accident,  provided that upon request by the  accused, the prosecution in a criminal  case shall provide reasonable notice in  advance of trial, or during trial if the  court excuses pretrial notice on good  cause shown, of the general nature of any  such evidence it intends to introduce at  trial.


9
The government provided notice in advance  of trial, and Williams does not challenge  the adequacy of that notice. Rather, he  complains that the district court  conducted only a cursory review of the  proffered evidence, and that the  admission of this evidence led the jury  to convict him based solely on his bad  character. The prejudicial effect of the  evidence, he contends, greatly exceeded  the probative value.


10
We review rulings determining the  admissibility of evidence under Rule  404(b) for an abuse of discretion. United  States v. Wiman, 77 F.3d 981, 984 (7th  Cir. 1995). The district court applies a  four-part test to determine the  admissibility of 404(b) evidence: (1) the  evidence is directed toward establishing  a matter in issue other than the  defendant's propensity to commit the  crime charged; (2) the evidence shows  that the other act is similar enough and  close enough in time to be relevant to  the matter in issue; (3) the evidence is  sufficient to support a jury finding that  the defendant committed the similar act;  and (4) the probative value of the  evidence is not substantially outweighed  by the danger of unfair prejudice. United  States v. Asher, 178 F.3d 486, 492 (7th  Cir. 1999), cert. denied, 120 S. Ct. 359  (1999). The only prong of this test that  Williams cites is the last one, and so we  will focus our attention there as well.


11
The district court engaged in a careful  analysis of the evidence the government  proffered, and decided that, on balance, some of the evidence should be excluded.  Thus the court excluded evidence that  Williams' pit bull dog had dug up drugs  in his yard in the summer of 1997 or  1998, finding that the reputation of the  pit bull was such that there was a danger  the jury would be unduly prejudiced. But  the court ruled admissible evidence from  five different witnesses that they had  either seen Williams in possession of  crack cocaine or that they had personally  purchased crack from Williams in 1997 and  1998. The court found that this testimony  would establish Williams' method of  operation, his motive to conspire, and  the availability to Williams of drugs.  The court also determined that the  conduct was similar to the charged  conduct and close enough in time to the  charged conduct to be relevant. Because  these witnesses were making statements  against their own interest, the district  court found that the evidence set forth  in the government's proffer would be  sufficient to support a jury finding.  Finally, the court found that the  probative value of the evidence was not  outweighed by the prejudice. In order to  minimize the prejudicial effect of the  evidence, the court offered to give  limiting instructions, and Williams  accepted this offer. The court gave the  instructions twice, once when the  witnesses testified, and again in final  instructions.


12
We assume that jurors follow the court's  instructions, unless there is substantial  evidence to the contrary. United States  v. Hernandez, 84 F.3d 931, 935 (7th Cir.  1996). Moreover, we accord great  deference to the district court's  assessment of the evidence because of the  judge's first-hand exposure to the  evidence and because of the judge's  familiarity with the case and ability to  gauge the impact of the evidence on the  jury in the context of the trial. Asher,  178 F.3d at 494. Because the court  properly applied the four-part test and  observed numerous safeguards to reduce  the possibility of unfair prejudice, we  conclude that the district court did not  abuse its discretion in admitting the  Rule 404(b) evidence.


13
Williams' final challenge is to the two-  point sentence enhancement he received  for gun possession under U.S.S.G. sec.  2D1.1(b)(1). That provision requires a  two-level enhancement if a weapon was  possessed during the commission of a drug  offense, unless it is clearly improbable  that the weapon was connected to the  offense. Williams complains that the  government's evidence in support of the  enhancement is "scant." The guns were  seized from a car parked at the home of  Williams' girlfriend approximately five  months after Williams was arrested, and  no drugs were ever found. The government  notes that Williams was living with his  girlfriend at the time of the seizure and  that this seizure merely corroborated  testimony from David Jackson, a co-  conspirator of Powell, that he had seen  Williams in possession of a number of  different guns during the time Williams  was dealing crack cocaine. We need not  decide if the government's evidence was  sufficient, however, because Williams  received the statutory minimum sentence  for a person with a prior felony drug  conviction who is subsequently convicted  of an offense involving in excess of 50  grams of crack. Williams does not dispute  that he was eligible for this statutory  minimum or that he in fact received the  statutory minimum. As a result, his  challenge to the two-level enhancement is  moot, and we need not decide the issue.  See United States v. Ivory, 11 F.3d 1411,  1413 (7th Cir. 1993). For all of these  reasons, the judgment of the district  court is


14
AFFIRMED.

