Lisa: High-toned voice...Bart, that's it! I know where Sideshow Bob is
[they start running]
When Bob broadcast that message, his voice was higher then normal.
And what makes your voice high?
Bart: Tight, binding underwear?
Lisa: Helium! Sideshow Bob is in the Duff blimp!
-- Well, they're both right,
"Sideshow Bob's Last Gleaming"