What is hidden in 3D image?

A philosophy professor stood before his class and had some items in front of him.
When the class began, wordlessly he picked up a very large and empty mayonnaise jar and proceeded to fill it with rocks, rocks about 2" in diameter.
He then asked the students if the jar was full? They agreed that it was.
So the professor then picked up a box of pebbles and poured them into the jar. He shook the jar lightly. The pebbles, of course, rolled into the open areas between the rocks.
He then asked the students again if the jar was full. They agreed it was.
The professor picked up a box of sand and poured it into the jar. Of course, the sand filled up everything else.
He then asked once more if the jar was full. The students responded with an unanimous - yes.
The professor then produced two cans of beer from under the table and proceeded to pour their entire contents into the jar - effectively filling the empty space between the sand. The students laughed.
"Now," said the professor, as the laughter subsided, "I want you to recognize that this jar represents your life. The rocks are the important things - your family, your partner, your health, and your children - Things that if everything else was lost and only they remained, your life would still be full.
The pebbles are the other things that matter, like your job, your house, and your car.
The sand is everything else. The small stuff."
"If you put the sand into the jar first," he continued, "there is no room for the pebbles or the rocks. The same goes for your life. If you spend all your time and energy on the small stuff, you will never have room for the things that are important to you. Pay attention to the things that are critical to your happiness. Play with your children. Take time to get medical checkups. Take your partner out dancing. There will always be time to go to work, give a dinner party and fix the disposal.
"Take care of the rocks first, the things that really matter. Set your priorities. The rest is just sand."
One of the students raised her hand and inquired what the beer represented.
The professor smiled. "I'm glad you asked. It just goes to show you that no matter how full your life may seem, there's always room for a couple of beers."