Hollywood's special effects teams can now endow extraterrestrial folk with convincing tentacles, steely teeth, and four quarts of mucus. Yet they often still look a little like the guy next door. Let's consider what good engineering practice can tell us about an intelligent extraterrestrial's appearance. To begin with, we have asserted that any worthy E.T. must be able to hold up his side of the conversation. Consequently, we can assume a high degree of complexity, in a biological sense.
Capability implies complexity.
To prove this point, note that in the late 1970s, microprocessor chips combined thirty thousand transistors on a single, small slab of silicon. These chips could handle simple word processing and modestly sized spreadsheets. The universal link between complexity and ability ensures that E.T. will be multicellular. This fairly obvious fact of intelligent life rules out the possibility that our alien pals will assume the shape of a malevolent mound of Jell-O, such as Steve McQueen confronted in The Blob. But of greater concern, the bulk of its protoplasmic innards would be situated far from its outer membrane.