I just found a weird thing in the garden. Best to explain with a picture

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Here’s the facts:

> it was 2.7 degrees C nearby as measured on my weather station

> the plastic cup was on the edge of a garden table – no nearby trees, overhanging branches or other stuff above it

> the cup hasn’t been moved for a while – at least 48 hours, probably longer

> the cup is completely full of water (now ice) – don’t know if it is just the surface or the whole thing that has frozen

> as far as I can see from the ice, there isn’t anything in it, just a line of bubbles (as you can see) from the water surface to the end of the ice stalagmite

I need help. WTF caused it?

I have one (very dodgy) theory which is that a drop of water fell into the cup right when it was at the point of freezing and left a kind of remnant, frozen droplet. But this is clearly shit unless I’ve forgotten some serious physics since university…

Suggestions, please

Update: I just found this post on New Scientist which explains this kind of phenomenon…it’s kinda complicated so I won’t try to precis it here…