Make glow in the dark (or under a UV light) goo out of Cornflour and tonic water
Cornflour, tonic water, cup and teaspoon. Suggest working at a wipeable table!
Add tonic water to a heaped tablepoon of cornflour gradually until it is just liquid. If you put the spoon in slowly it will be liquid but if you hit it it will be solid. Try pouring it onto your hand and rolling it! It will glow under UV light.