Take three different colour blocks, maybe red, yellow and blue.
Make a tower using one of each colour.
Here's one with red on top, blue in the middle and yellow on the bottom.

Now make another tower with a different colour on top.

How many different towers can you make?

When you are sure you have found them all, try it with four colours.

You may like to print off one of these sheets for recording three-block towers: Word document or pdf . Here are sheets for recording four-block towers Word document or pdf.

Click here for a poster of this problem.