Cerebrospinal fluid (CSF) is the liquid that surrounds your brain and prevents your brain from smashing in to your skull.
Here's a simple little demonstration that shows just how crucial that fluid is.
You'll need a small container (with a tight-fitting lid), 2 eggs and some water.
Throughout the demonstration, the egg yolk will represent your brain and the container will represent your skull.
Crack one egg into the container and put the lid on.
Agitate the container - you can just shake it, or you can have a student run a few laps around the room with it for it.
Open the container and observe - scrambled brains!
This time, before putting on the lid, fill the remaining space in the container with water.
Agitate the container once again.
Open the container and observe - the egg yolk remains intact. (The egg what froths up a little bit, making it difficult to see at first, but the whole yolk is there).
The water cushions the egg yolk, just as the CSF cushions your brain.
Great visual explanation!