The evolution of affordances on the web:
The URL for a page goes at the top. Text appears in a vertically scrolling column. A dropdown menu has a downward-pointing triangle next to it. Your mouse cursor is a slanted triangle with a tail, and when you hover over a link it looks like Mickey Mouse’s glove.
Most of these affordances don’t have any relationship to the physical characteristics of the interaction they mediate. But remove them from a website, application, or interface, and users get disoriented, frustrated, and unproductive.