I'm often asked "Why is the average mining time about 10 mins?". How do you answer this question if you are asked by beginners?
I'm not really against on Javier answer but i think it's still complex for a beginner to understand it.
Maybe give a realistic example of mining. "Real mining also takes time before you produce pure minerals out of stones which is the same with bitcoin mining, it takes some time before you receive fees from processing transactions."