Have you ever placed a 9-volt battery on your tongue? It's not very pleasant, specially when someone asks you to do it and you don't know what's coming.
On a serious note, the topic reminds me of an interesting book that I read; The Design of Everyday Things by Donald Norman, "is one of the classics of human interface design. Norman shows how badly something as simple as a kitchen stove can be designed, and everyone should read it who will design a dialog box, write an error message, or design just about anything else humans are supposed to use." --Qt Docs