It's always fun to play a trick on your eyes. They're so easily fooled! Take this illusion, for example. It's an old favourite. If you stare at the image long enough, the colours in the picture start fading away and eventually disappear. The colourful blobs shrink like evaporating water puddles
Just focus on the middle of the image and it'll start working its magic. For me, the red and purple go first, then the green and orange and then finally, the blue and yellow. If the colours start reappearing, focus harder and they'll eventually turn blank. It feels like you're controlling a superpower.