"What is like ice, but burns like fire?"
I think they got the riddle wrong in Turandot; the answer should be "Kozlik's mustard."
This was a must-stop during our Yelp tour of St. Lawrence Market. They have a bowl of pretzel sticks for you to sample about 10 different mustards. If you like sharp, intense pain shooting up your nostril, try the XXX Hot. My favorite was the Triple Crunch which starts off mild then burns slowly at the back of the throat. I also like the pop of the mustard seeds; kind of like a poor man's caviar.
Each 250-mL jar is $6; or four for $20. I only bought one jar but the nice mustard lady gave it to me for $5 because our Yelp group bought a total of four jars. Yay for group outings!
-------------------------UPDATE: January 18, 2010-------------------------
I made a mustard dipping sauce using Kozlik's Triple Crunch in place of the usual Maille whole-grain Dijon that I normally use and the S.O. says, "This sauce tastes better than the last time. Did you do something different?"
Yeah, Kozlik's.