Long, long time ago, in a far away galaxy, cells were formed. Cells are building blocks of different chemicals.

cells are building blocks of chemicals
Actually, you can make your own cells by boiling korean ginsang, dong guey, long mountain ginsang, and other vegetables or herbs together then leave the boiled material on the stove for three days, then you can see 1/2 cm diameter cells formed in a single line. (Yes these cells are big!) See picture below. This is what you should see in your pan.
This is what you should see in your pan

But why do cells unite in a single line? Well, a cell is also a magnet and has north and south pole just like a magnet. One cell's north pole attracts another cell's south pole, forming a single straight line.
A cell is like a magnet

Earliest animals were all worms united by cells in straight lines. Then a more complex type of animal started to show up.
round animal

The biggest cell in the middle is the most conductive cell in this type of animal. The biggest cell in the middle is the one which attracts and unites all other smaller cells to form into an animal body. Since it constantly attracts chemicals from nearby, weakest cells around this middle cell make way for chemicals to go in. This is the formming of the mouth.
formming of the mouth
Sonow, our very simple animal consists of a body and a mouth.