Easy, quick and simple to make- these are a great alternative to the rather pricey alternatives and can be customised however you like!! All you need is 2 parts sodium bicarbonate and 1 part citric acid. Mix well and add water sparsely until the consistency of wet sand, then mould in your hands of press into a mould, wait to dry and voila!!
Even more exciting with kids if combined with the red cabbage universal indicator!!