When Esk is older, Granny takes her to the Unknown University where wizards get their training. It is not easy because everyone knows girls can't be wizards, only witches, even though boys can be wizards and warlocks. Granny helps her get into the building but Esk shows her worthiness as a wizard, not by showing what she can do but more how doing nothing is sometimes the right thing to do.
As Esk tried to work out how to move the staff the ripples spread out in the magical ether, changing the Discworld in thousands of tiny ways. Most went entirely unnoticed. Perhaps a few grains of sand lay on their beaches in a slightly different position, or the occasional leaf hung on its tree in a marginally different way. But then the wavefront of probability struck the edge of Reality and rebounded like the slosh off the side of the pond which, meeting the laggard ripples coming the other way, caused small but important whirlpools in the very fabric of existence. You can have whirlpools in the fabric of existence, because it is a very strange fabric.
Pratchett has an amazing ability to turn words and the lesson I see in this book is not to assume. Anything. Ever. Not even how a sentence will end. Fabulous fun.
My rating for this book: ++++