This picture is taken from the Problem Pictures Themes resource of photographs with activities for mathematics teaching.
If you purchase this resource the photographs are available in higher resolution and there are additional pages containing hints and solutions.
When you turn over this egg timer, three sand timers measure 3 minutes, 4 minutes and 5 minutes. They cannot be turned over individually.
How could you use the timer to measure 6 minutes?
You could measure 7 minutes by adding together 3 and 4 minutes – but does it matter which you do first?
Is it possible to measure 2 minutes? Is it possible to measure half a minute?
What other times could you measure? What if you had three separate sand timers?