Re: Why does gelatin or Jell-O gel? What is the chemistry behind it?

Gelled gelatin is an entangled mess of protein chains, like a bowl of 
sticky spagetti.  When heated (the boiling water added to the Jell-O), the 
chains move freely and don't associate with each other, but as gelatin 
cools, the protein chains can't move as much, and they get stuck in a 
loose network.

Since this question is from a high schooler, here is a more advanced 
explanation:
Gelatin is a form of collagen, a highly structured, yet flexible protein 
that exists in animal's skin, bone and tendon tissues.  Collagen is a left-
handed helix of three protein chains -- somewhat like a DNA double helix.  
The chains are held together in the triple helix by hydrogen bonds.  When 
collagen is heated and denatured with acid or base, the triple helix melts 
and the chains come apart from each other.  The protein strands will never 
come together into the same, long triple helix as they were in collagen -- 
the conversion to gelatin is irreversible.  However, gelatin is thought to 
gel by renaturing in small regions to a triple helix.  Instead of the 
long, regular structure of collagen, gelatin is mostly free chains of 
protein, with short regions of triple helices holding the chains together 
in a network.  Again, hydrogen bonds are holding the triple helix together.

You should also check out another Madsci response written by a Cornell 
professor: http://www.madsci.org/posts/archives/jan99/916108681.Bc.r.html

This response was really written by Gelatin Expert and Risk Assessor 
Extraordinaire Siobain Duffy, who works in my lab, and who doesn't know 
how to spell extraordinaire, either.