The birthday boy/girl goes first at being the mouse.

He/she has to slowly walk around behind the children sitting down.

Then he/she taps one of the children on the head and says cat.

That child has to try and catch the birthday boy/girl around the circle before he/she gets back to his place and sits down.

If he/she does catch up, he tags the birthday boy/girl and says cat, he/she remains it. If they don't, the new cat is it, but is then the mouse.