Buttons are super useful, and the BBC micro:bit has two of them right here. One's called A and one's called B. There's a little label next to them that shows this.
These are your primary form of input. This is how you'll tell the BBC micro:bit to do something or start some action. Like "Hey, I just pressed this button, now blink me some LEDs!" Maybe one button can be "start" and one can be "stop". Or "faster" and "slower". Or "brighter" and "dimmer". It's up to you.
