A snail at the foot of a pole 10 metres high climbs 3 metres during each day and slips back 2 metres during each night. It starts climbing at the beginning of the first day. On which day does it first reach the top?

Each whole day-and-night gains the snail 3 − 2 = 1 metre, so at the start of day 8 it is 7 metres up. During day 8 it climbs 3 metres more and touches 10 metres — the top — so it never slips back again. The trap is the 10th day: dividing 10 by the net gain of 1 metre ignores the fact that the final climb is not followed by a night. Day 9 and day 7 come from making the same slip by one step in either direction.