You have to make them laugh. You must never underestimate the power of laughing in a maths classroom.
— Carol Roberts
quoted in Laughing lesson Adult learning
Or, as Donald O’Conner put it:
I think this is my biggest failing as a teacher: I am too much the straight man.
Denise Gaskins' Let's Play Math 
A good straight man is also a comic. Think George Burns. Letting the students tell the joke and make the punch line is probably better teaching than getting the laugh yourself.
Best dance scene ever, IMO.
Ah, but I didn’t say I was a *good* straight man.
Laughing is almost always a good thing. But the laughter could have something to do with illuminating a math concept. Costello’s method of fooling around with place value is one my favorite ways. See my non-youtube version:
http://bit.ly/aI2hEb
My lesson plan for it is here: http://bit.ly/9AP9lw