There’s a big bottle of shower gel in the cubicle, with a pump-action dispenser lid that’s screwed on. (Economy, see?)
It’s sat there, on the side, while I’m showering.
And I don’t think the lid fits perfectly—not airtight, at least—because during the course of a shower, bubbles start to form, around the base of the screw top. Happens every time I’m in there.