Five methods come to mind, which can be grouped by three fundamental human senses:
- Visual/Color: Change the color of the key which is pressed/touched (momentarily).
- Visual/Size: Increase the size of the key (again, momentarily).
- Visual/Animation: Animate the movement from the original size to the larger size.
- Audio/Sound: Make a clicking sound for each keystroke.
- Touch/Vibration: Vibrate the device to indicate that the key has been pressed.