I saw this idea here - we didn't exactly stick to measurements but this was the gist..
Combine 1/3 cup water, 2 tablespoons bi-carbonate of soda and a squirt or 2 of washing up liquid in one cup. Put this cup in a sandwich box or other container.
Mix around 5 tablespoons vinegar with a few drops of food colouring. We used red to start with.
Pour vinegar and food colouring into cup containing bi-carb etc.
Watch foam eruption happen and little boys amazement!
He wanted to do it straight away again so we used the same containers and added yellow colouring. We then did it again with green.
Just as we were finishing I got a phone call from our friends telling us that our local museum was having a science day that day. Perfect! We met them there and had loads of fun with all the experiments. The highlight was actually the fantastic slime experiment we had actually done before at home. Always a big hit! Basically the recipe is:-
In one bowl: mix 4oz pva glue with 4 oz water and 1 tspn food colouring
In another bowl: mix 1/2 cup water with 1 teaspoon Borax (this is a cleaning agent used a lot in the US. In the end I bought it v cheaply on ebay - a little goes a long way and has tons of uses so might be worth keeping in. NB Borax substitute found in Robert Dyas does not work)
Mix 2 bowls together and an amazing reaction occurs! Squish and drain off excess liquid and you will be left with wonderful slime. Keep in a zip lock bag and keep playing! Keeps for months - or until you get sick of it... does not come out of carpet!
We then had a adventure to the park on the way home!