Jello will eventually melt back to liquid form when left out of the fridge for too long.
When you remove the Jello from the fridge and eat it the Jello then melts in your mouth because your body temperature is warm enough to actually melt the Jello and turn it into Liquid.
So if the room temperature gets warm enough and the Jello Sits out long enough as in an hour or two hours or so then the Jello can and usually will turn back into liquid.
Then you would have to place the Jello back into the refrigerator for an hour or so to cool it back off and turn it back into the solid form.
But every time you eat the Jello it always turns back into liquid.