Could anyone help me explain this to a child aged 9, e.g. very simply? Many thanks...

If you have two sand-filled timers, one for 11 minutes and one for 7 minutes, how would you use them to time 15 minutes without wasting any time at the start?

If you start them at the same time, you would end up with one with 4 minutes left in it, but how would that work? Any help, explanations would be greatly appreciated (remember simple please)