Eye tricks
There is a trick to instantly discover the difference between these two images, which is what the contestant did in this TV show. You need cross your eyes so that the right eye views the left image and vice versa. It takes some practice to focus your eyes that way, but when you get it right you will see three images of coffee beans, where the middle one is sharp except the one missing bean will be half-transparent and easy to spot.
It is also possible to view so that your left eye views the left image and right eye the right one, but it’s somewhat more difficult for most people compared to crossing your eyes.
Other fun things you can do is put your palms in front of your head, one inverted so that their shape is similar. Then cross your eyes to merge them into one and walk around for a trippy effect. Someone also said that in boring meetings he tries to fuse faces of people sitting on the opposite side of the table.