We assign meaning to what we see based on our experiences and memories, and different objects can have equal meaning.
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Icons based on a familiar object like an envelope are particularly effective in illustrating an action, in this case emailing an article to someone else. Even a person who cannot read English would understand that the action represented refers to sending a message to a different person based on their experiences and memories of what an envelope means. There isn't even a need to have a standard icon for an envelope. The same is true of the shopping cart icon. It commonly related to completing a purchase. This translates to the web well because the cart's connotation is carried over regardless of whether the product being sold could literally be put in a physical shopping cart. Some websites use the metaphor of a shopping bag instead, and through isomorphic correspondence we easily assign the same meaning to these two different icons. It is not even always necessary to state "shopping cart" for users to understand what it is.