Pat-a-cake, pat-a-cake, baker's man.
Bake me a cake as fast as you can.
Pat it and prick it and mark it with 'S'
and put it in the oven for Sunbeams and me!

...and pat a cake Bakers man, so I will master as I can, and prick it, and prick it, and prick it, and prick it, and prick it, and throw't into the Oven.

The rhyme then appears in Mother Goose's Melody (c. 1765):

Patty Cake, Patty Cake, baker's man.
That I will Master as fast as I can.
Pat it and prick it and mark it with 'T'
and there will be enough for Tommy and me.

And if that wasn't fun fact enough...here's more:

It is thought that, back then, households that did not own an oven would make their pastry or baked goods at home and bring them to a local baker or bake house to bake for them for a fee.

Marking the pastry would have been a way to ensure the return of the proper item to the right household.

I shall never take my oven for granted again!