A standard marketing strategy for breakfast cereals and other products meant for children is to put some sort of "prize" at the bottom of the package--typically something plastic and useless, but still pretty cool to the target audience. Cracker Jack, having started this practice in 1912, was likely the first to do so. Since this strategy is specifically used to sell to kids (or, more accurately, their parents), it is most often seen referenced in cartoons, comic strips and other media that are considered as being meant for children, although that's not always the case.