Your Unit
Here’s how your Magic Wand is put together. Essentially, you’ve got a head and a shaft with 2 tunnels inside that run the entire length. These tunnels connect to tissues and heaps of blood vessels. When aroused and with the help of a naturally produced chemical, Nitric oxide, they fill-up with blood and you soon will find your Joystick at full mast… in other words, “We’ve got Bingo!!!”
The penis is made up of several parts including the glans, corpus cavernosum, corpus spongiosum, and the urethra.

What Is an Erection?

An erection starts in the brain. Something that has been seen, felt, smelled, heard, or thought makes the nerves send chemical messages to the blood vessels in the penis. Nitric oxide ( hyperlink ) is released to cause the arteries to relax and to open up to let more blood flow in; at the same time, the veins close up. Once blood is in the penis, pressure traps it within the corpora cavernosa. Nitric oxide again enables the penis to expand and hold the erection.

When the inflow of blood stops and the veins open, the penis becomes soft.  

Raising Your Flagpole
Mission Control, aka Your Brain, sends a signal to your Bad Larry via some chemical direct messaging (DM’s).  A thought, a smell, a sound, a touch… or some combination (if you are 15 years old, all it might take is a mild breeze) and the DM’s begin. This releases Nitric oxide and soon, it’s T-Minus a few seconds and your rocket ship takes flight.
FYI, Bonus Fun Fact:
Nitric Oxide is inside you… Nitric oxide (NO) is a colorless and odorless gas composed of one nitrogen atom and one oxygen atom. Its naturally produced in your body. However, Nitrous Oxide (N2O), is artificially manufactured and used to make cars run fast and furious and sometimes to even help rocket ships fly… they sound the same but are different compounds altogether, one natural and one for fast engines.