Onions, and by extension Shrek, have layers. Onions are a bulb, a sort of modified stem in a plant. Potatoes are tubers, which are modified roots and do not have layers. Potatoes are not onions. Shrek is not a potato.
So bulbs are special underground stems that some plants, like Shrek, can have. Therefore you can think of the layers of an onion are just leaves. Just like rings in a tree, bulbs grow layers as they grow older. The more layers an onion has, the older and more successful it has been in its life. Shrek must have lots of layers.
The purpose of a bulb is to store and protect water, nutrients, and other things Shrek needs to survive. This gives it an advantage over say, trees, which need a constant intake of sun and nutrients to survive. Onions like Shrek can store nutrients to use them when they need to.
The layers then also serve an evolutionary purpose. If a predator wants to eat the onion, it must first eat through many layers of icky tasting onion and will likely be discouraged. Of course, Shrek has no natural predators so this is not an issue.
Shrek’s layers are modified leaves which help protect the nutrients it stores inside. The more layers a Shrek has, the older and stronger it is.
If you’ve ever chopped up an onion, or looked at Shrek, you’ve probably ended up crying. Onions make you cry for the same reasons they taste bad. Onions are filled with enzymes, and when you break through their cells, with a knife or with your teeth, these enzymes are released.
Enzymes that were kept separated from the sulfenic acid by the cells are then free to mix, forming propanethiol S-oxide. Propanethiol S-oxide is a gas, which rises up and reacts with water in your eyes to form sulfuric acid. That burns your eyes, so your eyes produce more water to try to wash the acid away. So you cry.
If you want to protect your eyes from onions, you can cut the onion under running water (which washes the propanethiol s-oxide away before it can get to your eyes), or refrigerate it before you cut it. A cooler temperature will slow down the chemical reaction in general. If you want to protect yourself from Shrek, you can’t.
When you cut an onion, the natural enzymes mix with the water in your eyes to form sulfuric acid. Your eyes produce tears to try to wash the acid away.
The Average American eats 20 pounds of onion per year, so it would take 10-15 Americans to consume an entire Shrek in one year. There are 45 calories in a single serving of Shrek.