The blatantly obvious, it turns out, is generally only obvious in retrospect.
Madness string 81578
What with one thing and another, I hadn't played with toy ducks in my bath for years, and I found the novel experience most invigorating. For the benefit of those interested, I may mention that if you shove the thing under the surface with the sponge and then let it go, it shoots out of the water in a manner calculated to divert the most careworn.