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I always wondered about that

Need new gift ideas? Know a budding scientist? Then you need the New Scientist Gift Guide!

Also from New Scientist, the answer to a question you didn't know you wanted to ask:

Q: The two outer panes of a passenger aircraft cabin window have a tiny piece of cylindrical metal separating them. It is always near the base of the panes, not in the centre, and is frequently surrounded by condensation. What purpose does it serve and what is it made of?

A: Airline windows typically comprise three or more layers of glass (or acrylic) to provide insulation from the very cold atmosphere at altitude. The tiny silvery cylinder is really the edge of a small hole drilled in the middle layer to allow the pressure to equalise between the layers while minimising convection.

The condensation around the hole is due to the inner airspace cooling. Ice often forms here. The position of the hole is chosen to maintain the best clear viewing area when condensation forms, to minimise the likelihood of a crack forming between the hole and the edge of the window, and to avoid excessive condensation pooling over the hole, which could freeze and block it.