'Class, you guys must concentrate when studying. Do you hear me?' (rueful look, sombre tone)

'And do you know what concentration is?' (dramatic pause, lifting of right eyebrow) 'It's mass per unit volume or moles per unit volume. Get it?' (laughs hysterically as class stuns. stops after realising that no one else is laughing along)

*joke can be used when teaching Mole Concept, able to relieve tension and boredom
Here's another one: Most scholarships come with a minimum bond length.

'Do you know what's a bond length? It is the distance between the nuclei of the two atoms joined by a covalent bond, affected by the extent of overlap of electronic charge clouds.' (same laughter, same stunned expression, same sudden stop in laughing)
Think it'll be quite funny (to me). Most of my jokes are met with looks of utter disbelief from friends and family. Apparently, they don't find such puns funny.

Then again... don't think I'll ever display such personal humor to a class. Seems as though it's the surest and fastest way to gain a reputation for being a nutcase, hahah.