From a scientific perspective, jelly is considered a non-Newtonian fluid, which means its viscosity changes depending on the force applied to it. Yeah, it's a mouthful! But basically, it means that jelly can behave like both a solid and a liquid depending on how you interact with it.
For example, when you apply a lot of pressure to jelly, it becomes more fluid and will start to flow. But when you leave it alone, it will return to its solid-like state. It's like it's playing a game of "I'm a solid! No, I'm a liquid!" and it's pretty mind-blowing.