Because sender and receiver don't share the same word-symbol for the metal piece. In fact, the message it conveys to the receiver is understandable, but totally wrong. Be sure all the words in your message are understandable in the way you mean them to your audience. For example, an iron bar can only be a medium of communication if both parties agree on a common meaning for the sounds produced by banging on one end with a hammer. Smell is only a useful method of communication among dogs because they share a genetic memory of what those various smells mean.
Any message contains a core idea.
It's important to search for that core idea, and not get distracted by the secondary ideas and descriptive words which enrich the core idea. Failure to make it clear can result in anger and frustration by the receiver. For example, visually you could make it bold, make it move or blink, make it a bright color, surround it with white space, etc. Communication problems often arise because of assumptions. We assume that we share a common meaning for the word or picture symbols being used.