Eso sí, ¿qué es?

I remember hearing an advertisement for a program to help English speakers learn Spanish.  It was some sort of audio tape thing.  The advertisement was on the radio.  The catch phrase at the end was:  "Have you ever wondered how to spell 'socks' in Spanish?  ...  Eso sí, ¿qué es?"  That last part in Spanish sounds like you're spelling out S-O-C-K-S.

I remember that something clicked after I learning that the Spanish word for socks is los calcetines.  I asked my teacher about it because that would be spelled c-a-l-c....  She chuckled and told me about the pun.  I think it was a pretty clever marketing ploy that you wouldn't understand unless you didn't need the product in the first place.