Whimsy

Whimsy a capricious notion or fancy.

                          Deviation from the ordinary.   
 

Dainty Mice

"But why?"  asked Miriam, "why did all the mice get up and start dancing all at once?  There is no rhyme or reason!"

The professor stroked his chin thoughtfully, "It must have been a whim, a flight of fancy."

"Madness," muttered Miriam.