Weirdness with the MEGA LED

I have my Mega r3 on a wood, non-conductive surface, yet the LED on pin 13 always seems to do its own thing when not programmed. Sometimes it flashes, sometimes it fades, sometimes it does nothing.

Most strangely, when I touch the bottoms of the header pins, or when I jostle the board about, the led blinks or grows brighter, and completely goes out when I touch any of the gnd pins.

Why is this happening? It’s not damaging anything, but I’d like to know why.

You have pins that are floating. Set the pin as INPUT_PULLUP