To whip cream, you need chilled butterfat (around 35-40%) called whipping, heavy, or double cream. As you beat the cream, it forms bubbles and the proteins denature, with some parts staying in the water and some parts staying in the fat, until you end up with a film of solid fat and protein that traps the air inside, with the water in between the bubbles. If you beat the cream too much, you can turn the whole htingthing inside out, with the water trapped indideinside films of fat and protein, and the air gets out. (turns runny). Don't add sugar or flavor until you start getting a stiff whip or the sugar will disrupt the trapping of air bubbles. To get a stiffer peak or cream, you can add a small amount of unflavored gelatin to the cream as you first start to mix and whip.