
The left slipper has a sound chip that makes a "space invaders gun firing" noise if you press the red label; the right has four LEDs, representing a shot from the defending thingy hitting the invader, which flash if you stamp your foot or press down hard. I've had to fake this by touching in the red on a couple of the LEDs, it wouldn't photograph properly in five attempts.
And they're pretty comfy, which is nice.
They come from Marks and Spencers, no idea of price but they may turn up cheap now Xmas is past.