I was trying to capture images of two small diving ducks -- perhaps female hooded mergansers -- on the river this morning. Whoops, where did they go? Only a spreading whirlpool to show that something ducked under the water. (Hey, that prompts a quick etymological query, and sure enough it looks as if the verb duck is indeed related to the word for the bird. Or vice-versa, actually, as it appears that the origin is a word meaning "to dive.")
Oh, I know she was here a moment ago!
Hey, this time there are TWO little whirlpools showing where BOTH ducks were a second earlier!