This from my friend Matt's eleven year old son Alex. The little sod solved it in his head. It took me an hour and a half, two pieces of paper and two pens:
You got a garden.
There is a mole.
It has dug five molehills in your lawn, in a line connected by a tunnel.
The mole is under one.
You hit a hole with your hammer.
If the mole is under the hole, it dies.
If it is not, it moves either left or right.
Call the hills 1 2 3 4 and 5.
The mole is under hill 2. You hit 3. It moves to 1. You hit 4. It moves to 2. You hit 1. It moves to 1 ......etc.
Can you kill the mole? How many whacks?