This is a classic:

In Konigsberg, Germany, a river ran through the city such that in its center was an island, and after passing the island, the river broke into two parts.  Seven bridges

were built so that the people of the city could get from one part to another.  A crude map of the center of Konigsberg might look like this:

The people wondered whether or not one could walk around the city in a way that would involve crossing each bridge exactly once.

Try to ‘plan your journey’ with a pencil in such a way that you trace over each bridge once and only once and you complete the ‘plan’ with one continuous pencil stroke.