[URL="http://www.blacklibrary.com/Blog/Whats-20-feet-high-has-two-heads.html"]Here[/URL]:
What’s 20 feet high, has two heads and shoots lightning from its hands?*
Jumbo version available at the BL site linked above.
*Don't say 'Your mum', don't say 'Your mum', don't say 'Your mum'...