A string of impossible ifs, wishing beggars into riders and turnips into watches, that lands on a proverb about wishing rather than doing. Older children tend to get more out of the joke than younger ones.
From Mother Goose / Nursery Rhymes (traditional). See the whole collection.
If wishes were horses, beggars would ride.
If turnips were watches, I would wear one by my side.
And if "ifs" and "ands"
Were pots and pans,
There'd be no work for tinkers!
Public domain. Text from The Real Mother Goose (Blanche Fisher Wright, 1916), via Project Gutenberg. View the source edition