a shopping cart having knobs , ridges or other protrusions upon which plastic store bags can be hung by their handles . it may also be thought of as the method of providing such a cart to a shopper or shoppers , and may include the further step , performed by a shopper , of hanging store bags from the protrusions . the invention may also encompass a shopping cart having bag hangers that are positioned and adapted to retain a tray , and the method of positioning a tray on a shopping cart .