You do burst cells when you sit down on broken glass or a pile of pins. The experience is painful and messy. But let's assume you were referring to sitting on a flat chair, in which case you do indeed burst very few of the cells in your body. Why is this? The answer lies in the difference between sitting on broken glass and sitting on a flat chair. When you sit on a flat surface, your entire weight is spread over the large surface area of your rear. There are millions of cells per square inch of skin, so the weight that each individual cell experiences when you sit down is actually quite small. This effect is one of the great advantages to having a body made out of very small, self-contained subunits. In contrast, when you sit on an upright pin, much of your weight gets focused to just a handful of cells and they do indeed burst. Sharp objects are so good at cutting because they deliver a moderate total force over a very small area.