I woke up this morning, got out of bed and walked out of the bedroom into the hall. I was immediately struck by a strange shimmering stippled pattern of light on the hall wall, whose cause wasn't immediately obvious.

I walked over to it and had a closer look. It was a pattern of spots of white light in a roughly hexagonal lattice, each one varying slightly in brightness over time but not moving in space.

I looked around for strange-shaped light sources … and all became clear. The light source was the sun shining through the blobby glass[1] in my flat's kitchen window; the kitchen door was closed, and the shape of the blobby window was being projected through the keyhole in the door to make a perfect upside-down pinhole-camera image of the kitchen window on the hall wall. Cool!

(I'd have taken a photo, but by the time it occurred to me the sun had moved on and the image had faded. Shame.)

[1] Is there a proper name for that, incidentally? The kind of glass you put in bathroom windows so that it lets in light but doesn't let out an undistorted image of what you're doing in there. Sometimes it's simply frosted glass; other times it has a pattern of distortion of some sort. The stuff in my flat has hexagonally tiled round blobs. There must be a name for the general concept of glass-for-bathroom-windows-and-the-like, but I couldn't find it in a quick trawl of Google and Wikipedia.