The other day I received a letter from a friend. My love for letters is very old. These magical spacetime capsules bring in thoughts of a person written in a different location about a different period of time in their lives. And while reading that piece of paper, one can slowly be transmitted to that zone, as if by a time-machine — a becalming feeling very different from the disturbing ting-ting you get when you receive a one-line WhatsApp message on your phone.
An important part of the letter writing ritual, however, is to buy stamps of your choice and fix them on the envelope. Stamps are magical things in their own right. To affix one, you only need to moisten the blank side with a few drops of water (or, if no one’s watching, your spit) and it’s ready to stick.
Why is it so simple, though?
Forces of nature
Any two things stick to each other due to attractive forces between them. These forces can arise due to multiple reasons. For example, when you jump, you fall back to the earth. What brings you back? That’s the gravitational force, one of the fundamental forces of nature. The heavier the object is, the stronger the force is.
How much force does the whole earth exert on any of us? That quantity is what we measure as our weight. A 60 kg of weight means about 600 newtons (N) of force. If you’re worried about your weight, it’s just the earth pulling you towards itself with that much more attractive force.
Another important force we see around us all the time is the electromagnetic force. It contains two types of forces that are interrelated: electric and magnetic. Electrical forces happen due to electric charges. We know electrons are negatively charged while protons are positively charged, and electrical forces arise between them. Similarly, magnetic forces occur between magnetic charges that we call north and south. Like charges repel while unlike charges attract.
When refrigerator magnets stick to the fridge door or electric forces from your battery drive a current in your fan, it’s all due to electromagnetic forces.
There are two other fundamental forces, called the weak and strong forces, but they are only important at very, very small distances, like inside the nucleus of an atom. We don’t experience them in our daily lives.
The question now is: which of these fundamental forces keeps the stamp stuck to the envelope?
World of glue
This question takes us inside the world of glue. What we call glue is really a chemical compound called polyvinyl-acetate (PVA). That name may not say much: it’s really a material with long, chain-like molecules of carbon, hydrogen, and oxygen. These molecules look like noodles and tend to stick to each other — and also to other surfaces.
Atoms are usually charge neutral because they each contain an equal number of positively charged protons and negatively charged electrons. Every molecule, which is made of multiple atoms, is then also charge neutral. We know that like charges repel and opposite charges attract. The puzzle here is to figure out how one charge neutral molecule can attract another charge neutral molecule.
The answer has to do with a beautiful piece of physics discovered in the early 1900s. Even though every molecule is charge-neutral overall, it can fluctuate such that one part of it can be slightly more negatively charged than another. And when such a molecule comes close to another charge-neutral molecule, it can slightly distort the distribution of charges in the second molecule.
Thus the molecule, which was until recently overall charge-neutral, now starts feeling attracted to another charge-neutral molecule — all because they have opposite charges at different parts of their bodies. This kind of force, while it’s secretly of the electromagnetic variety, goes by the name Van der Waal force. It’s named after the Dutch physicist Johannes Diderik van der Waals.
This brings us to the final piece of the puzzle: why do we need those few drops of water?
Swimming in water
Imagine you want to stick one piece of paper to another. You’ll find the glue bottle and inside you’ll see that it’s a liquid. This is important.
It turns out that manufacturers mix the PVA molecules with water to prevent them from fully sticking to one another. This way, the PVA is swimming in a small pond of water.
Since these molecules attract each other due to fluctuating charges, they can stick to each other only if they’re relatively close. But if there’s enough water around them, these molecules will swim far apart and not feel each other’s attraction.
On the other hand, when you take some glue and spread it around a surface, the water evaporates, bringing the PVA molecules closer to each other. Imagine there are a bunch of people in a swimming pool: you can make it more crowded by reducing the pool’s size instead of adding more people. It’s the same principle.
If you’re quick enough, you can bring the two pieces of paper you need to stick together before the glue dries. This way, the PVA molecules will stick to both surfaces as well as to each other, sealing both pieces of paper together.
The glue will thus have done its job.
Quantum stamps
The same glue trick works on stamps as well — just that the back side of a stamp already has dried glue. A few drops of water just makes these molecules separate out and become more dilute, allowing you to place the stamp on the envelope. Then, the water molecules dry up again and allow the stamp to stick to the envelope.
In all this, you might have wondered: what causes the changes in the PVA molecules to fluctuate, leading to the Van der Waals forces? Understanding this requires us to learn a bit of quantum mechanics. If you’re interested, you should consider taking a course in physics, for example here in IIT Kanpur, where some of us teach.
The next time you receive a courier or a letter from a friend, don’t just thank the person bringing it to you. Also whisper a word of gratitude to the molecules hard at work behind the stamp, and which probably also kept it safely sealed all the way to your doorstep.
Adhip Agarwala is an assistant professor of physics in IIT Kanpur.
Published – September 23, 2025 08:30 am IST