Sand-filled timers questions

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)

Re: Sand-filled timers questions

Start both timers at the same time. Once the 7-minute timer is empty, flip it. Four minutes later, the 11 minute timer will be empty, but only 4 minutes of sand will have emptied from the 7-minute timer. So, if you flip the seven minute timer just as the 11-minute timer empties, it will only have 4 minutes of sand in the top. That is a total of 11+4 = 15 minutes.