Jello is a colloid. A colloid is a type of mixture where one substance is dispersed evenly throughout another. In the case of Jello, the protein molecules from the gelatin are dispersed in water. This creates a unique state that has properties of both liquids and solids.
When Jello is in its liquid form before it sets, it behaves like a liquid. However, once it sets, it takes on a more solid-like form, but it is not a true solid. The gelatin molecules form a network that traps the water, giving Jello its semi-solid, wobbly texture. This is why Jello is classified as a colloid rather than a pure liquid or solid.