All you need is some Lego blocks, and a muffin tray.
1) Take the Lego blocks and put them in the tray, one stone per hole.
2) Set the oven at room temperature and let your kid put the tray anywhere in the oven. make sure he wears his oven glove!
3) Leave it in the oven for 1 minute.
4) Let your kid take it out of the oven again.
Enjoy your Lego!