It's brightly colored, and made from rendered animal skins and bones....
Nah, just kidding. (Not about the animal bones--that's absolutely true.)
Your startup is like Jello because it starts of in a liquid state, but undergoes a phase transition and becomes (more or less) a solid.
When your startup begins, it is completely fluid. You can change just about anything, practically overnight. It won't even count as a pivot, because a pivot requires that you establish a solid position from which to pivot.
This fluidity is what makes startups powerful. Unlike established companies, they aren't tied to the traditional ways, and can better serve new or changing markets.
But your startup loses much of that fluidity even before you become the Evil Empire. As soon as you start taking in money from investors and customers, your sloshy soup of collagen firms into a jiggly semi-solid.
And if you've ever tried to reshape Jello, you know that it's possible, but difficult (and your hands are likely to get sticky and stained in the process).
Make sure you've found the right mold for your startup *before* you raise money, or you might end up stuck with an unappetizing Jello salad.