You have two egg-timers -- a four-minute timer and a seven-minute timer.
These are hourglasses which contain sand.

Explain how you can cook an ear of corn for 9 minutes. (Tell when you would turn over the timers, when you would start cooking the ear of corn, when it would be done, etc.).



Solution:

There are several solutions to this problem.
Here is one of them:

Start cooking the corn and turn over both timers.
After the 4-minute timer runs out, turn it over.
After the 7-minute timer runs out, turn it over.
When the 4-minute timer runs out the second time, turn over the 7-minute timer (which has 1 minute of sand in it).
After the sand runs out in the 7-minute timer, 9 minutes will have elapsed and the corn will be done. 


       7 min                   1 min 1 min 
  |----------------------------|----|----|

       4 min          4 min
  |----------------|----------------|

                9  min
  |<------------------------------------>|


Correctly solved by:

1. Tom Marino Winchester, VA
2. Jia Ran Rome, Italy
3. Court Pifer Winchester, VA
4. Bill Brown Winchester, VA
5. Chip Crawford Winchester, VA
6. Jonathan Pence Winchester, VA