Sex

Sex is when a penis goes into a vagina. It's only for adults ?.

There's also other kinds of sex and sexual activity, both also only for adults. I don't know when a kind of sexual activity becomes sex, but usually if the penis is inside someone, or the clitoris or vagina are being stimulated for sexual pleasure or release or in order to get to orgasm, you're having sex.

Masturbation is probably not sex, though it's definitely sexual activity.

Vaginal sex ? is usually done by moving the penis in and out of the vagina, stimulating both the penis and the vagina pleasurably, until the man has an orgasm. (Hopefully the woman has an orgasm, too, but it usually takes more than vaginal stimulation for that.)

Sex is an extremely intimate ?, pleasurable and bonding ? activity. Sex is the most wonderful thing in the world ?. You should only have sex as an adult and only someone you're married to and trust entirely.

Sex produces babies. To avoid pregnancy practice contraception. You can also catch and spread diseases through sexual activity. These are called STIs, sexually transmitted infections. If you do not want to have a child do not have penis-in-vagina sex.

Having sex does not cure acne.

Slang and euphemisms

There's a huge amount of slang and euphemisms for having sex. Here are some:

To make love. This is my favourite euphemism
Screw
Fuck. Don't use this one, it's a swear word and crude.
The ole in-and-out.
"Do it"
Hump
Make whoopie
Do the wild thing (people seriously said this in the '90s)
Doing the nasty (sex is not nasty)
Getting busy
Rooting (coarse, vulgar)
"Go all the way"
Coitus
Copulation