If you play organ or piano, or any other instrument with the same type of keyboard, here's a simple test to determine your powers of observation. But first, to make absolutely sure that there's no cheating at all, I must ask you to step away from your instrument. Is it completely out of view? You're quite sure? Good!
Ok, I want you to imagine you've opened a music shop and you're going to stock spare parts for your instrument, and in particular, spare keys for the keyboard. It's fairly obvious that any black (ie sharp) key is just the same as any other black key, so you'll only need to stock one pattern of black key to cover all eventualities. The question is this: how many patterns of white (ie natural) key will you need to stock? For the purpose of this exercise, you can ignore any special keys at the extreme ends of the keyboard, and just think about the regular white keys which are repeated in each octave. When you think you know the answer, scroll down to see if you're correct ...
... keep going ....
...just a little bit more ....
... and the correct answer is .... 7. It's true, no white key is exactly like any other. If you look closely at G and A you will see that neither is symmetrical. The front of G goes farther to the right and the front of A goes farther to the left, so they are not interchangeable with each other, nor with D which is symmetrical. And because C is made to nestle snugly against symmetric D, it is not interchangeable with F which nestles against asymmetric G. Nor is E interchangeable with B. I admit I would never have spotted this myself if I hadn't taken a MIDI keyboard apart and noticed that the white keys were numbered 1 to 7, which at first seemed to me to be quite unnecessary.
So, full marks if you said 7. Your powers of observation are probably well above average. If you gave the "common sense" answer of 3, you can have half a mark for effort. All other answers score a big fat zero, I'm afraid, but don't feel too bad about it. So far, everyone I've tried this test on - including a professional piano teacher with years of experience - has got it wrong!