A beautiful simile for something fitting perfectly into a circular space.  In this case, the something is a someone, the marvelous Mij, an otter celebrated by Gavin Maxwell. 

Mij mastered taps and baths, and it was hard to keep him away from them.  

Mij had an instant eye for anything connected with water, and the most cursory inspection of the sleeping compartment convinced him that in the wash basin, however dry at the moment, lay the greatest pleasure-potential; he curled up in it, his form fitting its contours as an apple fits a dumpling, and his paws began increasingly feverish experiments with the chromium tap. 

Source: Gavin Maxwell, Ring of Bright Water (London: Penguin, 1974), p. 115

Photo credit: Schnu1 at pixabay