Timer for showers?

I have two teenagers who love to stand in the shower and run the water until the hot water has been drained from the hot water heater. Anyone know of an in-line device that could be installed either before or after the water heater that could be set for a predetermined time that would then cut the flow of hot water thus forcing them to end their shower?

I'm sure this device could make a mint!!!

Coin Operated Timer

Do a web search for a coin operated timer and use it to operate a solenoid valve on the hot water. Charge 5 or 10 cents per minute for hot water.

It's called mom goes to the water heater and slowly shuts off the hot water. They will get the message after a few cold water showers.

By the Time they get out you are where you should be and they don't know.

Or get a coin operated one like Bob NH suggested.

I just shut off the hot water. Wait about 60 seconds, waiting for the yell "DAD". They get the message. I usually have to do it a few times in a week. The problem seems to subside for a time.