You dont 'tell' your eyes to cross, you just look closer or further away. Try looking at the image at normal distance, whilst imaginging that you are looking into the distance at a beautiful view or at the horizon on an ocean. It is this difference in distance focusing which causes the illusion.
Actually, given enough practice, you can literally just get your eyes to do it. I started doing magic eye puzzles as a kid and loved it, so just eventually learned how to control my eyes in that way. Even today, if I see any repeating pattern, or even anything vaguely similarly shaped, I can’t help but do it
Well actually, I've been telling my eyes to cross since I was a child. I can't describe it, it's like tensing a muscle in the eyes or something and you can control the angle with the tension.
I can even rotate my eyes! Did you know we have muscles for that? I trained it in the mirror - try tilting your head and look at your eyes REALLY closely: they rotate a bit to cancel out the tilt.
Whenever I try to do this, the most I get is that the two images touch. The cats in the example are holding paws, but they never overlap. I've been trying to make this work since the old magic images from the 90s, but I've never managed it. I wonder if there isn a hardware limitation related to my eye configuration.
