Here is the Interactive NRICH shape sorter.

It has three holes: a triangle, a square and a circle.
Here are three shapes which need to go into the shape sorter: a triangle, a square and a circle.

This text is usually replaced by the Flash movie.

Click and drag each shape to move; click on the arrow to rotate.

How many different ways can you post each shape into the sorter?
What does this tell you about each shape?

A black dot has been drawn on each shape to help you keep track of the different ways of posting them.