An engineer and a mathematician were walking in the forest, one day, when they come to a cottage in a clearing.

In the cottage there is a bundle of wood beside the fireplace, some old newspapers and a box of matches. There is also a bucket of water, a kettle, a teapot, cups and some tea bags.

The engineer suggests they light a fire, boil some of the water in the kettle and make a pot of tea.

The mathematician agrees.

After they do all this they drink the tea and head off walking again.

A few hours later they come to another identical cottage. In this one the fire is already lit and the kettle is boiling.

The engineer suggests they use the boiling water to make tea.

The mathematician disagrees.

Instead he suggests they use some of the water to put out the fire. This will return them to the situation they met in the first cottage – it will get them back to a problem they have already solved, and they can use the solution they know that worked.