Great question Leah! Jelly, is also known as gelatin – a substance formed when collagen protein (material that connects tissues together) is submerged in boiling water. As it dissolves, the collagen bonds are broken down, but once it’s cooled down the molecules of collagen have a tendency to aggregate and thicken. Luckily, as the mixture cools in the fridge, some of the bonds do not reconnect…. causing jelly to jiggle! These gaps fill with water, giving jelly it’s wobble and wiggle signature. Water molecules becoming trapped between collagen molecules is very much like trapping water inside a balloon. You’ll notice the similarities in fluid motion of gelatin and a water balloon – that’s just how water moves when confined to a given space. Hope that helps to answer your question 🙂
Nice answer Angela! And did you know Leah, that some fruit can’t be made into jelly at all – pineapple for example contains enzymes or chemicals which can break down the collage bonds required to make the jelly set!
An easy way to test this at home is to make some gelatin/jelly plates, and then add a slice of pineapple on top of one plate. After a while, you’ll see that the pineapple makes the jelly turn into liquid!