A shower thought is not simply "a thought you have in the shower", it is actually written in the sidebar:
Miniature epiphanies you have that highlight the oddities within the familiar.
That's why folks will reply with "not a shower thought", and I agree with them.
A "Showerthought" is a simple term used to describe the thoughts that pop into your head while you're doing everyday things like taking a shower, driving, or just daydreaming. The best ones are thoughts that many people can relate to and they find something funny or interesting in regular stuff.