A tickling game dressed up as a test of good manners: whoever laughs at a tickled knee fails to be a gentleman. It reliably gets a giggle, which is rather the point.
From Mother Goose / Nursery Rhymes (traditional). See the whole collection.
If you are to be a gentleman,
As I suppose you'll be,
You'll neither laugh nor smile,
For a tickling of the knee.
Public domain. Text from The Real Mother Goose (Blanche Fisher Wright, 1916), via Project Gutenberg. View the source edition