A MATTER OF TIMING

You have two regular sand timers, each of which takes a different amount of time for the sand to flow through:

The first sand timer takes 5 minutes for the sand to flow through.

The second sand timer takes 7 minutes for the sand to flow through. Using these two timers, and nothing else, how can I measure the passage of exactly 9 minutes?

Turn over both timers together, and when the 5 minute one finishes turn it straight back over. When the 7 minute one finishes, 2 minutes will have flowed through the 5 minute one. If you now immediately turn it back over again, then those 2 minutes will flow back. When complete, that’s 9 minutes.