S for Stupendous!

From one of my favorite moments in the Calvin & Hobbes comics...

S for Stupendous!
T for Tiger, ferocity of!
U for Underwear, red!
P for Power, incredible!
E for Excellent physique!
N for ...um... something... well, come back to that...
D for Determination!
U for... wait, how do you spell this? Is it "I"??