At the physics exam...

'Describe the universe in 200 words and give three examples.'

A student recognizes Einstein in a train and asks :
Excuse me, professor, but does New York stop by this train?

While solving a numerical :
The answer to a problem was 'log (1+x)'. A student copied the answer from the student next to him, but didn't want to make it obvious that he was cheating, so he changed the answer slightly to 'timber (1+x)'

Charge less :
A neutron walks into a bar; he asks the bartender, 'How much for a beer?' The bartender looks at him, and says 'For you, no charge.'

Positive and Negative :
Two atoms bump into each other. One says 'I think I lost an electron!' The other asks, 'Are you sure?', to which the first replies, 'I'm positive.'