We want more...

A professor sets an aquarium tank onto a table in front of his class.

He proceeds to fill it with a bucket of big rocks.

After filling the tank with all the big rocks, the professor asks his class, "Is this tank full?" 

The class nods.

The professor then takes out a bucket of pebbles and fills the tank with them. The pebbles fall into the crevices between the big rocks. 

He asks his class again whether the tank is full. 

As before, the class nods.

The professor grins and takes out a bucket of sand. This bucket of sand fills the crevices between the pebbles.

"Is it full?" he asks.

The class doesn't respond.

Finally, the professor takes out a bucket of beer, and fills the tank with it.

Having done all this, the professor looks at his entire class, and asks, "What's the moral of this story?"

A student responds, "There's always room for beer."