Suppose you have two hour-glasses, the ones that have sand in them. One will time 7 minutes and the other will time 11 minutes. Also suppose that you are going to make a phone call that must last exactly 15 minutes. (That's a lot of supposing, but whatever.) Now figure out how to use nothing but the two timers to time your 15 minute phone call. (Yes, yes you could look at your watch, but play along with this.)
Solution
Start both timers together. Start your call when the 7 minute timer runs out. There will be 4 minutes to go on the 11 minute timer. When it runs out, run it again for another 11 minutes, and hang up when it runs out.