Why didn't the lysosome function when it saw the net equation of photosynthesis?
It was too basic.
Why was the Latin verb telling everyone what to do?
It was in a bad mood.
What's the difference between a rotating metronome placed on a stack of worksheets and a stressed student?
The metronome always turns in assignments on time.
That last one is slightly depressing, so I think I'll get back to studying now. Things are going well, but I'll be glad when these exams are finished.
UPDATE: Have to add this gem from someoldjokes.tumblr.com:
From Cambridge Jests: Or, Witty Alarums for Melancholy Spirits by a Lover of Ha, Ha, He, 1721
That book title, though.