Now here's a mystery for you. I thought it was a - well, I wasn't sure what it was, but it had no right to be besmirching my bathroom window, so I nudged it with my finger. (I interrupt this mystery story to point out that when I say bathroom, you shouldn't picture a room with a bath in it.
I am using the word in the North American sense, as a euphemism for, well, for what? For toilet or lavatory or WC or washroom or cloakroom or little boys'/girls' room or powder room or indoor outhouse or john or garde-robe, every one of which is already a euphemism in its own right. For when it comes to avoiding admitting we have a digestive system, we're so astonishingly squeamish that we squeam all the way to the thesaurus.
Bathroom isn't my usual euphemism. I was brought up to say toilet, but somehow "toilet window" didn't quite seem to say what I wanted it to say. It seemed to suggest a viewing pane of glass set into the vitreous enamel which was not what I meant at all, so I said bathroom instead.
That I can do so without either gagging or giggling illustrates only how pervasive American English has become in the last half century. For I can still remember first going to North America in the early '80s and being brought up short by sentences such as "I laughed so hard I went to the bathroom in my shorts".
It was on the same visit that I first came across an even more spectacular lavatorial euphemism on a door in a McDonalds restaurant in Seattle. Rest room, it said. I had a momentary image of obese Americans recovering from their daily burger surfeit by reclining on a chaise longue and sipping at a little soda water.
Rest is a serving euphemism in another context, of course, indeed it has been employed for centuries with reference to the most euphemised subject of all. Come with me and my dog round the local graveyard and you'll see that every third headstone bears the phrase "At Rest".
Superficially the words seem apter for the dead than for users of the lavatory, since the dead are at least lying down and doing nothing - or were when last seen. But the point about resting is that having done some you get back up and go about your business once again, which the dead are defined by not doing. The dead are dead. Which is of course the upsetting truth that causes us to reach for euphemism.
Sleep's popular on headstones too. "Just sleeping" they say, especially of people buried young. Understandable perhaps but evasive of the truth nevertheless.
And sleeping brings us to the third big subject for euphemism: sex. When two people are said to sleep together the point of the phrase is the antithesis of sleeping. Indeed if you mean sleeping and nothing else you need another verb.
Start compiling a list of euphemisms for the business of reproduction and the parts of the body associated with it and any one of us could fill a page in five minutes.
So there they stand, the three great horses of the Troika of Euphemism, the huge unspeakables, the unholy trinity of the truth we try not to acknowledge: sex, death and defecation, and I think we can all agree they'd make a winning name for a rock band.
Why is it that we struggle to admit we are as the other beasties are, that we eat and mate and die? Perhaps we are just too fond of ourselves. For we love to tell ourselves stories of our own importance, of our dominion over the beasts of the field and the birds of the air, to see ourselves as the beloved creations of a father figure who bears an uncanny resemblance to ourselves. Do dogs do the same? I don't think so. So why we should is a mystery.
Which brings us back to the thing on the bathroom window. It proved to be a beetle of sorts, half an inch long. When nudged it fell to the windowsill on its back, writhed to right itself then scuttled off. A vase stood on the windowsill, a vase of mottled pattern, greys and browns and blacks and paler bits. The beetle scaled the vertical side of the vase - how is a mystery for another day - until it found a smudge of colour that was exactly the same colour as itself and there, precisely, it stopped still and effectively disappeared. The mystery: how did it know?