A detachment of soldiers must cross a river. The bridge is broken, the river is deep. Suddenly the officer in charge spots 2 boys playing in a rowboat by the shore. The boat is so tiny, however, that it can only hold 2 boys or 1 soldier.
Still, all the soldiers succeed in crossing the river in the boat. How?
Answer
First the boys cross the river. One stays ashore while the other brings the boat to the soldiers and gets out. A soldier gets in the boat and crosses.
The boy who has stayed there brings the boat back to the soldiers, then takes the other boy to the other shore. Again a boy brings the boat back, gets out, a second soldier crosses over. This process is repeated until all the soldiers have crossed.