Simply put, a shopping cart is a metal cart on wheels, used for temporarily storing items (such as groceries) prior to paying for them.
Shopping carts made their first appearence around 1937 in Fort Lee, VA. The first shopping carts were simple folding metal frames with handles and wheels. Hand-held baskets could be placed on the carriers, wheeled around the store to collect items, and removed at check-out. Unfortunately, the "folding basket carriers" as they were called, did not catch on very quickly. Young males during the time thought that by using the carts, they would appear weak. The young women of the time considered the carriers to be "unfashionable", and the older people thought it would make them appear helpless. To encourage the use of the carts, grocery store owner Sylvan Goldman (who first introduced the cart) hired people of all ages, both male and female, to push the carts around the store. With the "models" pretending to shop using the carts combined with an attractive store greeter who encouraged their use, they began to catch on.
There have been very few technological breakthroughs in the design of the shopping cart in the past fifty years or so, due to the simplicity of the design. Unfortunately, the traditional metal shopping cart will eventually suffer from a bent frame, causing the entire case to warp in a strange, yet subtle manner. This causes the wheels to start spazzing out, flipping back and forth in either direction. A shopping cart with a proper frame will have its wheels locked in whichever way you're pushing the cart. Unbalanced and misaligned shopping carts are difficult, and sometimes impossible to control properly.