Two lines long and built for a knee-bounce: three straws propped on a staff, said to make a baby laugh and cry in the same breath. About as short as a nursery rhyme gets.
From Mother Goose / Nursery Rhymes (traditional). See the whole collection.
Three straws on a staff
Would make a baby cry and laugh.
Public domain. Text from The Real Mother Goose (Blanche Fisher Wright, 1916), via Project Gutenberg. View the source edition