We’re familiar with optical illusions, but they’re not the only kind. Every one of your senses is subject to illusions.
The “knobby sphere illusion” is an example of an illusion of the sense of touch.
You can do it yourself. You only need two things: An ordinary pencil (the six-sided kind) and a marble or other small, hard sphere. Close your eyes. Squeeze the sides really hard for about one minute until the pencil makes dents in your skin, then roll the marble in your fingers. The marble is smooth as ever, but it will feel weirdly bumpy.
The bumps are in your own skin, of course. If the skin surface that’s sensing a shape is distorted, the way it senses will also be distorted.