Sometimes mistakes make for the best inventions! When Stephanie at the Two Da Loo made the fabulous galaxy slime, she made one batch that was too hard. By adding more liquid starch, she ended up with Gak, which is not as stretchy as slime. But, this can be used to blow bubbles — reusable ones! Check out her post for the tutorial.
Science lesson: Adding more starch equals more crosslinking of the polymer chains in the glue, making a harder substance.