Firstly eat a packet of crips. You need to wash out the packet and make sure it is dry.

Cut out a hole in the top left hand corner, about the same size as a penny.

Put on a tray and shove in the oven on full heat, make sure you keep and eye on it so that it dosent shrink too much or melt. It only takes about 1-2 minutes

Take it out and let it cool.

It should go really hard and now run along and put your key on it!