Because when two things rub against one another, it makes friction -and friction usually makes things hot. So when you rub your skin you are making heat.
Slipping and sliding
Friction not only creates heat, it also helps stop things from slipping. The rougher things are, the more friction there is when they rub together. Things that are smooth or wet are slippery because they give less friction. That's why you're likely to slip when walking on ice in shoes that are smooth on the bottom.
Friction facts
- If you rub two sticks together for long enough, the wood will get so hot it will start a fire.
- Brakes on a bicycle use friction to slow the wheels downs.
Ice skating
When ice skaters want to stop, they push the jagged part of their skate blade into the ice, which creates friction.