How flies fly?

I make no apologies for being sad enough to have even clocked this in the first place, but I’ve clocked it nevertheless, so must find the reason behind it.

Why is it that flies appear to particularly enjoy flying round underneath ceiling lights / fans? They regularly buzz about underneath them, sometimes with a couple of buddies for company.

Also, when they are buzzing about under the said light / fan, they always seem to change direction very sharply - never in a smooth, curved pattern.

Why is this? :confused:

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