What does marijuana actually do to your body, as in long and short term side effects, that i can explain to my 18 year old nephew who will not quit?

Marijuana: In the short-term, mj can cause feeling of euphoria and increased appetite, which are not necessarily bad things. But it can impair your ability to perform tasks requiring concentration like driving a car, make you sleepy with red eyes,  reduced social inhibition and reduced short-term memory or anxiety. Long-term side effects include infertility due to reduced libido and low testosterone.