No dumb blond
As generally happens with these events, the lecturer wanted a volunteer from the audience to help solve a problem. Down bounded a child with glorious long golden locks.
"What's your name, young lady?" enquired the lecturer. The moppet blinked. "John," he replied. "CUT!" yelled the director, and young John was sent back to his seat for an action replay.
Take two: a volunteer was called for, John was chosen and again bounded down the steps. "What's your name, young man?" beamed the lecturer. John blinked. "I TOLD you 10 minutes ago," he replied. It's a good idea not to work with animals, either.