You want to find out how quickly your friend rides from home to work on his bicycle. However, you don't have a vehicle, so he is faster than you. You cannot be both at the starting point and at the finishing point of the trip, and you don't trust him enough in this important matter.
You and your friend both have stopwatches. How can you reliably measure the time in which your friend can ride to work? (You have to measure a one-way trip, two-way isn't good enough -- even though it is uphill and against the wind both ways, a two-way trip is a longer route).
Published in Njet Problem! collection 1996-09-24.