How can you make a gooey substance?

Dough. Google for recipes. The most basic dough is just flour with a bit of water. Don't forget to knead. It goes bad in a matter of days, but if you add *lots* of salt, it becomes much less perishable.

Apart from that, it has nothing but advantages. It's harmless when swallowed, you can tweak the properties from very sticky (some bread doughs) to very tough (pizza dough) and when you're sick of it, you pop it in the oven and make food with it (provided you didn't try to conserve it by adding lots of salt) ;)