Why is the aorta considered to be the "mother of all arteries"

The aorta is considered to be the "mother of all arteries" because the aorta is the largest blood vessel in the body. 

If it's a smaller child or infant, place one on the middle of the chest and on the child's back. (if it is a bigger child with enough room for two, place one on the upper right/your left of the chest, and one on the lower left/your right) the pads should never touch each other.