After a pinata party, Maria e-mailed me the following question:
Hey, why does water with flour become glue for pinatas? Is there a scientific explanation for the stickiness? How come it doesn’t work like that when it makes bread? Is bread sticky too? I’m really confused!
Off the bat, I knew that flour has starch and the starch makes the glue sticky, but then I started wondering too. Why is starch sticky? And why isn’t bread dough more like glue?
Starch is the major component of flour. Starch exists in tiny granules, which swell and break when boiled in water. This releases the starch molecules, which then all stick together to make a goopy, gluey mess. 
You can make starch glue from flour and water, by mixing one cup of flour in some cold water, and adding this mixture to 3 cups of boiling water. You can even use less flour, and it will still work. 
Starch glue is good at sticking paper to paper, to make pinatas for example, because paper itself contains cellulose, which is quite similar in structure to starch. They’re both polysaccharides. You can’t use starch glue to glue plastic or metals – only other polysaccharides.  (In making pinatas, the first layer of paper sticks to the balloon. I think this works because while the glue is still wet the wet paper will stick to the balloon. Once the glue is dry, it actually doesn’t stick to the balloon anymore, but at that point the paper has been permanently shaped around it.)
Like starch glue, bread dough also contains water and flour. Still, it’s not as sticky – you can’t use bread dough to glue paper together. Let’s look at the ingredients for a super simple recipe to find out what’s different.
Ingredients for bread
1 c. milk (Hot)
1/4 c. sugar
4 T. margarine
1 tsp. salt
1 pkg. yeast
4 c. all-purpose flour
1/4 c. warm water
Instead of 1 cup (or less) of flour and 3 cups of water, we now have 4 cups of flour and 1.25 cups of warm water and milk. (The rest of the ingredients don’t add a lot of water or starch, so I ignored them, although they will make a difference to the final dough: the yeast, for example, makes the dough rise.)
The ratio between the flour and water is completely different between glue and dough. Would this be crucial in whether or not it’s glue?
According to this website on the science of bread ingredients, the amount of water is indeed the answer:
In bread making not as much water is added as when making a sauce or gravy, and gelatinisation isn’t completed – the starch granules swell, and many don’t burst to form a gel. This forms a network of bloated starch granules all touching at the edges. 
So there you go: water and flour glue is sticky only for paper and other cellulose-based materials. It’s the large amount of water that makes it sticky, because it releases the starch from starch granules. Bread dough doesn’t have that much water, and therefore isn’t as sticky.