One of n bottles is poisoned, and a rat that drinks from it dies after the test period. What is the fewest rats that can identify the bottle in one round?
Number the bottles in binary. Rat i sips from every bottle whose bit i is 1. After the test, the pattern of dead rats spells out the poisoned bottle's number in binary.
With k rats you can tell apart 2ᵏ bottles, so the answer is the smallest k with 2ᵏ ≥ n, which is ⌈log₂ n⌉. Each rat is one bit of information.