In uk.d-i-y, Martin Angove wrote:
Boy won't pick a sweet up
from the pavement, but might well be allowed to recover a slice of apple
dropped on the living room carpet.

This kind of distinction can usefully be rationalised as the "five
second rule". If it's been on the floor or pavement for more than five
seconds, you're not allowed to eat it. That rule is simple enough for
young children to remember, and gets the right answer pretty well most
of the time.

--
Mike Barnes