picture description: There is an object composed of different parts.  The part on the very bottom is probably hollow and has something slightly smaller than the inside so that it can slide inside.  The part that slides in and out of the part (let's call it a box) is called a drawer.  On top of the box is a metal contraption we call a scale.  The scale part often is a separate but in this case is attached to the box below (the drawer part).  The scale has a bar at its top that rests on a fulcrum.  Hanging from the bar are two baskets on either side of hte bar.  In each basket you can put things.  If the mass of hte item is great than the in the other the basket will be pulled down low until it would come to rest on the top of the box.  The act of placing objects in the baskets and seeing which has greater mass is called balancing.