I have two young cats (a male and a female, litter mates, 19 months old) who have developed an intriguing behaviour that I haven't seen in any other cats before.
Without fail, when I come downstairs in the morning, one of them has dropped a toy mouse into their (ceramic) water bowl. They don't do it with other water receptacles; they don't put anything else in there (no food, no other toys). And although they mostly do it during the night (when there's nobody awake to pay them attention) they also do it during the day. The water in their bowl is changed twice daily (morning and night).
I don't think it's attention seeking behaviour, but to avoid reinforcing it, I'm removing the toy from the bowl when they aren't around.
And I don't think they're bored -- they're indoor cats, but they have a selection of cat trees, chase toys, boxes with holes to make them work for their food. And of course a hard-working slave who will play fetch with them in the evening for hours on end, or chase the light-pointer.
It isn't a problem (except maybe for the repeatedly wet toy mice), unless it's a sign of something I shuld be worrying about, but it is puzzling me.
Edited to add (in response to a comment): These are the 19th and 20th cats that I've had, and I've never encountered this with any others, hence my puzzlement. They don't do it any other fabric items that they play with (socks) or drag around (duvet covers, draught excluders).