In a closed room, including, a floor, 4 walls, and a ceiling, a fly wishes to rest little. Put on the wall of the bottom, it tracks down a spider, situated contrary to this room, on the opposite wall.
Knowing that the spider walks at the speed of 1 metre / minute, it decides to sleep 30 minutes and 10s, because, it thinks, the shortest road which separates it from the spider measures: 8+22+1 = 31 metres (road represented in red).
Thus it begins to sleep, but exactly 30 minutes later the spider eats it!

How the spider dones that?

Note: The spider cannot jump, either drop, or weave a web, or make some acrobatics.