The Answer
Jell-O is a matrix of loosely-connected solid material (collagen) suspended within a liquid (water). This structure is referred to as a colloid.
In high school we learn that there are three primary states of matter; solids, liquids and gasses (plus plasma). Jell-O doesn’t seem to fit neatly within any of those exactly.
A solid is a form of matter characterized by a particular shape that also has volume. A liquid, on the other hand, has volume but no exact shape, taking the shape of its container.
Jell-O does take a particular shape and does have volume, so perhaps it’s a solid. However, before it sets, it takes the shape of its container and is unquestionably a liquid. Apply a little pressure to Jell-O and it starts behaving like a liquid again.
So, what is it?
Jell-O is actually something called a colloid but a colloid is not a separate state of matter. It is effectively a matrix of solid material suspended within a liquid.
Colloids occur when a substance is microscopically and evenly distributed throughout a second substance. Colloids are one of three types of mixtures; the other two are solutions and suspensions.
Jell-O is made with gelatin and water. This gelatin is a collagen protein that comes in a powdered form. When the gelatin is mixed with water it becomes become suspended in the water and when the water is it becomes it dissolves.
The magic happens when that hot water/Jell-O solution cools. The protein molecules harden back into solid form. These solid particles bind with other nearby particles forming a three dimensional structure. The liquid water molecules are trapped in this grid of loosely-bound solid proteins.
When you eat Jell-O, you disrupt and warm up this loose structure in your mouth and it becomes nearly liquid again. Yummmm!