There are several recipes for making slime. The easiest one that can be done by a 3-years old is Elmer's glue - Borax mixture. Making slime is fun and not as messy as you may think, even when 13 kids are making slime at the same time...
To make things easy, I diluted Elmer’s glue with water at 1:1 ratio in a large pitcher and made a super saturate borax solution in advance. Each kid received a small bowl containing the diluted glue solution, food coloring and a small cup with the borax solution. First they added the color to the glue and observed that the glue is a liquid. Then they added the borax solution and mixed it to create colorful slime. The slime was very sticky at first, but after kneading it became nice and pliable.
The 4-5 year old kids enjoyed adding water to the slime and watch it become more slippery as it absorbs the water.
The slime can be kept in a sealed Ziploc bag for a long time and is fun to play with. Like play dough, slime can be molded into different shapes, however, it does not retain it's shape for very long.
The science of slime:
The mixture of Elmer’s Glue with Borax and water produces a putty-like material called elastomer. In simplest terms, polymers (like in the Elmer’s glue) are made out of long strands of molecules like spaghetti. When a pile of freshly cooked spaghetti comes out of the hot water and into the bowl, the strands flow like a liquid from the pan to the bowl. Similarly the long polymer molecules slide past each other easily then the substance acts like a liquid because the molecules flow. If the molecules stick together at a few places along the strand, then the substance behaves like a rubbery solid called an elastomer. Borax is the compound that is responsible for hooking the glue’s molecules together to form the putty-like material.