Though all hockey rinks have the fundamental shape of a rectangle with rounded edges, ice rinks come in a variety of shapes and sizes. NHL size and Olympic size are standard-sized rinks that differ by 15 feet in width. Recreational arenas vary in size and are often smaller than professional rinks. There is no set length for a hockey rink because the game can be played on a half-sheet of ice if necessary. The key difference between an ice rink and other forms of skating facilities is the presence of ice cubes. In order to play on an ice rink, there must be water present, even if just a few inches. This water can be natural such as a pond or lake, but it can also be man-made such as a tank or pool. Skaters use various techniques to control their speed and direction including jumping, twisting, and turning of the body. They also use sticks to poke holes in the ice to create paths for themselves to move forward. Ice skates were originally made from animal bones and later wood. Today's blades are mostly made of carbon fiber although some skaters prefer using metal blades due to their weight. The ends of the blade are sharp for cutting through the ice and sometimes have serrations (notches) for extra grip. There are two types of goals in ice hockey: straight-ahead and roundhouse.