There's something kind of magical about being able to make your phone or tablet touchscreen do stuff just by swiping your finger or tapping your thumbs. But you might be surprised to find out a banana could work just as well as your finger.
If you stick your finger on a regular piece of glass, the most you can hope for is a smudge. So how does wiping your finger on a glass touch screen make things happen inside your phone or tablet?
Our futuristic touch screens rely on a bit of physics that's almost as old as Isaac Newton — the ability of a system to store electric charge, known as capacitance.
That's why bananas can also swipe right — they conduct electricity about as well as your finger.
And if you put on a glove (on your finger or on the banana) you won't see any effect. This is because gloves are made from material which doesn't conduct electricity.
Pens won't work either, for a different reason. They do conduct electricity, but they do it too well.
But anything that's got about a finger's worth of free-moving charge — like a humble banana — will do the trick.
That's the simple version of how touchscreens work, but it's actually a lot more complicated than that.
Journey to the centre of the smartphone
The part of your smartphone that detects touch sits above the LCD screen and the battery and circuits.
It's made up of two sheets of glass and a bunch of wires that are so skinny they're see-through.
The top sheet of glass is the one you touch — it's mostly for protection and it keeps your finger away from the business end of things.
The second glass layer has skinny wires running above and below it. The wires run in a grid — horizontally on one side of the glass, and vertically on the other side. And they're hooked up to the battery — one set of wires to the positive terminal, and the set on the other side to the negative terminal.
Your smartphone switches on — and off — every possible pair of wires. It does this over and over, in the same order each time. And it does it so fast it happens many times each second.
In every possible pair of wires, the one that's hooked up to the battery's positive terminal gets electrons sucked out of it, and the negative terminal pumps electrons into the other wire. That difference in charge causes an electric field between the two wires, and it's strongest where the wires cross over.
So each point where these wires intersect has a tiny electric field — these electric fields are really small, but they still affect nearby charges — like the electrons in the layer of glass.
Glass is an insulator — its electrons are held tightly by its atoms, so they're not free to flow as an electric current. But the electric field between the wires pulls the electrons a little bit towards the positive wire. No current flows, but pulling all those electrons closer repels electrons in the positive wire, and attracts more electrons from the battery to the negative wire. So the positive wire gets a bit more positive and the negative wire gets a bit more negative, and that means the electric field gets stronger.
Strong enough to reach out of the top of the phone.