It has to be intellectual stimulation!
A couple of years ago I was talking to a theoretical physicist who works at CERN. The guy was short, overweight, bearded and in his 60's... but by the time he'd explained to me why there have to 13-Dimensions in the Universe, I was just about ready to rip his clothes of there and then
Talk physics to me baby, oh yeah!
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"May the bridges I burn light the way" ~ Anon
"Be happy for this moment. This moment is your life." ~ Omar Khayyám
"I mean, yes, we're sinking. But the music is exceptional." ~ Anon (1912)