At the age of six or so, I was friends with a girl in my class who also lived in my neighborhood. We would spend afternoons alone at her house, playing in her room. I don't know how it began, except that I remember lots of anatomically confused, childish 'dirty jokes' that older kids would tell on the school bus. We began to act these out, in a way. Fully clothed, we would lie on the floor behind her bed, on the carpeting, and wriggle against each other until our thighs were firmly locked together. We would roll over and over, and argue over who got to 'be the girl' (open her legs around the other one's hips, which felt better), but we'd always end up each humping the other's thigh. We would do this for hours, grinding our crotches together and writhing on the floor. I can't speak for her (though she almost always initiated it) but I loved the way it felt, tingly and tantalizing and delicious.
Ten years later I had a boyfriend. He was a nice boy my age, a virgin like me, and we had sexual tension to spare. One weekend we went camping with his parents. Doing anything at night was out of the question, but during the day we snuck away into the fields. He rolled on top of me and began rubbing his denim-covered bulge against me, the warmth and hardness melting a wet spot on my own jeans within seconds. He kissed the hollow of my throat, I ground against him and he began thrusting harder against me, losing control for a moment. He stopped himself, but that rubbing was really beginning to get to me and the last thing I wanted was for him to stop. I went for his earlobes, which I knew drove him crazy, and tweaked his right nipple lightly for good measure. His hand slithered around behind my ass and he pulled me to him so tightly our hipbones ground together. His hips worked furiously, and I matched his thrusts with my own, and soon we were both quivering and moaning and clinging to each other in post-orgasmic bliss.