HOW COME: Why glue sticks -- to most things

How come glue doesn't stick to the inside of its own bottle? asks Kameryn Gosine, a student in Brookville, NY.

Glue is a mystery, seeming to stick to everything from paper and plastic to our own fingers and (ouch) hair. Everything, that is, except its own trusty bottle.

But even before there were bottles of white glue to buy every school year, nature made its own adhesives. From egg whites bonded to a dirty breakfast plate to that gummy resin oozing from last month's Christmas tree, human beings have a long history of glue-y annoyances and sticky fingers.

Some adhesives work by seeping into the cracks and crevices of two surfaces, sticking them together like burrs stick to shoelaces. This is known as mechanical bonding. If an adhesive's molecules are attracted to the molecules of the two surfaces to be attached, that's intermolecular bonding. An adhesive can also fuse with the molecules on the surfaces, a process called chemical bonding.

From your own gluing adventures in crafting or repair, you may have noticed that two roughly textured surfaces will tend to bond more strongly than two satiny-smooth surfaces. Which is why bottles or other containers that hold glue are made of smooth-surfaced (impermeable) materials, helping the glue inside to stay unstuck from inner walls.

However, it turns out that packaging material isn't the main reason why glues don't stick to their bottles. Enter another culprit in adhesion: the air around us.

Many adhesives are converted from gloppy liquid to hard solid by air-drying. Such glues contain not just the sticky substance itself, but a solvent that keeps the glue liquid until it's applied. In everyday white glue, the solvent is water. In other glues, the solvent may be a volatile organic chemical like acetone or toluene.

Water evaporates much more slowly than other solvents, which is why white glue dries more slowly than, say, the plastic cement used to glue models together. When the glue dries completely, the two surfaces are bonded.

According to the glue manufacturers, glue hardens when the solvent it contains evaporates into thin air. So as long as the glue remains inside an airtight bottle, cap firmly screwed on, it should remain a gooey liquid.

What about air inside the bottle, lurking in the space between the glue and the cap? According to glue makers, there's a natural limit to how much of a glue's solvent can evaporate into the tiny volume of air in the bottle. In fact, once that small space is full of, say, water vapor, no more water will evaporate from the pool of glue. So the glue stays moist and elastic, rather than hardening into an unusable lump.