You basically all types of forces everyday you open your eyes, for example gravitational force (keeps you from floating into space) or frictional force (used when walking, your going forward at a certain rate of speed) or applied force (when you push a shopping cart at the market, your applying that to the cart)
He sold one half of the pie because if you do it in your head split one half into 3 and get one sixth, then that slit in two is 1 twelfth so if he sold six pieces and six is one half of twelve then you get one half.
