2.4. A man lies dead in a room with fifty-three bicycles in front of him.
2.4 answer: The "bicycles" are Bicycle playing cards; the man was cheating at cards, and when the extra card was found, he was killed by the other players.
2.4a. Variant: There are 53 bees instead of 53 bicycles.
2.4a answer: The same (Bee is another brand of playing cards).
2.4b. Variant: There are 51 instead of 53.
2.4b answer: Someone saw the guy conceal a card, and proved the deck was defective by turning it up and pointing out the missing ace. Or, the game was bridge, and the others noticed the cheating when the deal didn't come out even. The man had palmed an ace during the shuffle and meant to put it in his own hand during the deal, but muffed it. (both answers from Mark Brader)