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.
Public domain. Text from The Real Mother Goose (Blanche Fisher Wright, 1916), via Project Gutenberg. View the source edition