How Do You Spell The Plural of Mississippi?

I had to urinate for the majority of our hellish ninety minute ride to Queens last weekend, and by the time I got into the bathroom my bladder was ready to rupture, so I decided to count how long the stream lasted (without trying to extend the time by constricting the flow) and I urinated for 63 "Mississippis"-- and to put that in perspective, I had to whiz pretty badly yesterday after teaching three classes in a row and it lasted 20 "Mississippis."