Goo is a solid in the Solids menu. It is element number 12.
Goo is a dark brown solid, which acts like normal. It is not flammable, nor it doesn't conduct electricity. It is not affected by gravity or heat.
However, under pressure, Goo seems to spread and diffuse. Their life decreases. If enough pressure, Goo particles' life will decrease to zero and the particles die.
Goo is used to pressure walls, and for animations (since Goo's life will decrease automatically if not in it's stable form). In animations, goo's life is set with console or PROP. The one with less life is removed first with the animation.