Disappearing water

Today S5 played a trick on Mrs Taylor from the office.  We put some of a nappy inside, in a polystyrene cup and pretended it was empty.  We then put water in it but had to leave it for a while so it would swell.  We then poured it over her head but nothing came out.  The inside of a nappy is polymer and it absorbs water and swells like a jelly.