Why Is Glue Sticky?

WHY? is our attempt to answer all the questions every little kid asks. Do you have a question? Send it to why@mentalfloss.com.

There are different kinds of glue. Let's talk about the glue you probably use in school—white glue. Glue is adhesive, or sticky. It is also cohesive. That means it sticks to itself. So how does glue do that? Glue is made of polymers (POLL-uh-murrs), which are big molecules (MOLL-uh-kyools) made up of smaller molecules all joined together. The polymers that make up glue are strong, flexible, and spreadable. These qualities help it to stick to paper, plastic, glass, wood, and other surfaces.

Glue also has another important ingredient: water. When you squeeze a glue bottle and the glue comes out, air touches it. That causes the water in the glue to evaporate, or change from a liquid to a gas. When the water evaporates, only the sticky polymers remain in the glue. Without water, the glue dries, gets hard, and keeps your artwork in one piece. That’s called mechanical adhesion (muh-CAN-ick-all add-HEE-zhun). Guess what? Glue doesn’t dry out inside the bottle.

If you close a glue container up tight when you’re finished with it, there will be very little air inside. Since the water in the glue is not exposed to much air, it doesn’t have a chance to evaporate. That keeps the glue from drying out. There are many kinds of glue, and different glues work in different ways. Scientists are always looking for new and better types of glue. They have discovered that some natural glues even work under water!

Some amazing marine animals like mussels make their own glue. That’s why mussels can stick to rocks even underwater. This natural glue is different from regular white glue that comes in a bottle. It is made of proteins. Scientists have learned how to copy mussel glue and mix it with something called biofilm (a kind of slime made by bacteria) to make an even stronger glue. This glue is so powerful that it can help fix ships under water and help heal wounds without using stitches. Pretty cool, huh?

Want to make your own glue? Check out this recipe. And for more sticky facts about how mussels make their own glue, watch the video below.