Fission chips.
They have the ability to penetrate both holes at the same time. explanation(http://en.m.wikipedia.org/wiki/Double-slitexperiment)
The mathematician says "2" The Physicist says "2, plus or minus 0.1" The engineer says "Probably around 2, but let's say 3 to be on the safe side".
Because he wanted to drink two beers with his lunch.
Lunch is on me."