Anyone who has typed on a mechanical keyboard knows the sound this makes. The keychain is a row of four real-style mechanical key switches under clear keycaps, joined in a transparent housing, with a metal chain and clasp at one end. Press a key and you get the crisp tactile click of a blue switch, and the key lights up blue from underneath.

It is built to be clicked hundreds of times a day. The switches have a firm bump halfway down and snap back up, which is why gamers and programmers keep one next to the laptop and why students keep one on their bag. Because the whole thing is clear, you can see the switch stems and the springs working inside, which children find as interesting as the clicking.

The blue light comes on as each key is pressed, so tapping out a rhythm across the four keys makes the row flicker. In a dark room or a car at night that becomes a game of its own.

The chain and clasp clip onto keys, a school bag zip, a pencil case or a belt loop, so it goes wherever a fidget is needed. It is compact: the four keys together are about the length of an adult finger.

One honest warning for parents: it clicks. That is the point of a blue switch, and it is satisfying, but it is not a silent fidget, so it is better for the bus ride than the exam hall. Keep it dry, because there is a small light inside.

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