Sexual intercourse is when a boy's hard PENIS goes inside a girl's VAGINA. A sexual relationship will probably involve this but may also involve other sexual acts too.

Sex usually starts with both the girl and boy "getting turned on" with lots of kissing and touching. This is called FOREPLAY. Without foreplay the girls vagina may not get moist enough (ouch!) and the boy may not get an erection. Then they both move their bodies so the penis moves up and down inside her vagina. This usually results in the boy having an orgasm. Depending on the position you are in, the girl may have an orgasm too, if the movement rubs her CLITORIS.