Ermm yeah if ya want to

It's similar to this other one I've heard...

A science teacher is showing a simple experiement.
He has a glass container into which he puts some pebbles, right up to the
top and turns to the class, "Is this container full?" he says.
One boy shouts out "Yes"
So the teacher gets out some gravel and starts to pour it into the
container, shaking it so that it moves down inside the gaps between the
pebbles - when he's done, the level is up to the top again.
"Now is it full?" he asks
The boy shouts out again "Sure it is, I can't see the gaps anymore!"
So the teacher gets out some sand and pours that in, again filling to the
top.
He turns to the class and says...
"Now it is full - The lesson here is that you can't always see everything so
you need to investigate further"

A second boy pipes up "It's not full!" - He walks to the front of the class,
takes a can of beer from his bag, opens it, and pours it into the
container - He gets quite a lot in and gets the level up to the top.
He turns to the teacher and says...
"Now it is full - The lesson here is that there is always room for Beer!"

Matt