A riddle in three lines: soft as silk, bitter as gall, wrapped in a green coat. Children enjoy guessing before being told the answer is a walnut.
From Mother Goose / Nursery Rhymes (traditional). See the whole collection.
As soft as silk, as white as milk,
As bitter as gall, a strong wall,
And a green coat covers me all.
About this rhyme
Checked against real walnut anatomy: kernel (soft as silk, white as milk), the thin bitter skin around the kernel (bitter as gall), the hard shell (a strong wall), and the green outer husk (a green coat). All four map correctly onto the rhyme's four images.
Another riddle built entirely from description, this time of a walnut: soft kernel, bitter skin, hard shell, green husk. Children can often guess it faster than adults, especially if there is a real walnut to hand while you say it.
Public domain. Text from The Real Mother Goose (Blanche Fisher Wright, 1916), via Project Gutenberg. View the source edition