3D was good
I'd say you could do with studying the mechanics of how a trampoline works. I know it may be getting a little technical, but if you understood that better, then maybe the animation would look a little better. Put some springs around the edge - they stretch, not the fabric of the tarpaulin.
When the cow jumps, make more than just his knees bend, as if he's really putting some effort into getting into space. Arms, hips and knees should all be moving. These little touches would make it so much better.
Also, a better sound effect for the trampoline itself and maybe a slightly quieter music track.
Other than that, it'snot going to set the portal ablaze, but it'll certainly be a passable flash animation.
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