Gelatin
Gelatin is a protein powder that traps water when it cools, turning liquid into a wobbly solid like Jello.
Think of it this way
A clear glass bowl holds warm fruit juice. Tiny protein bits float free in the liquid. As the bowl cools in the fridge, the protein bits link arms and form a loose net. The net holds the water in place, turning the juice into a soft, wobbly solid.
Explaining gelatin by grade level
When you make Jello, the powder mixes into hot water. As it cools in the fridge, it turns thick and wobbly. Fresh pineapple juice can break the Jello apart. Something in the pineapple stops it from getting firm.