I actually had a similar question on my test and this is the answer:
The reason the magnet loses its magnetic properties is is because when you drop the magnet or bump it against something, the domain can get bumped or dropped out of alignment.
Hoped this helped :)
Have a great day <span />
His name in the show is kelso
13-13= 0, 19-13= 6, so then 0-6= -6