Wizards had always known that the act of observation changed the thing that was observed, and sometimes forgot that it also changed the observer too.

He’d always felt he had a right to exist as a wizard in the same way that you couldn’t do proper maths without the number 0, which wasn’t a number at all but, if it went away, would leave a lot of larger numbers looking bloody stupid.

Unseen University was much bigger on the inside. Thousands of years as the leading establishment of practical magic in a world where dimensions were largely a matter of chance in any case had left it bulging in places where it shouldn't have places. There were rooms containing rooms which, if you entered them, turned out to contain the room you'd started with, which can be a problem if you are in a conga line.

Any true wizard, faced with a sign like 'Do not open this door. Really. We mean it. We're not kidding. Opening this door will mean the end of the universe,' would automatically open the door in order to see what all the fuss is about. This made signs rather a waste of time, but at least it meant that when you handed what was left of the wizard to his grieving relatives you could say, as they grasped the jar, 'We told him not to.