I don't want to spoil this for you by telling you what to expect.
Download this file (scan it for viruses of course) and unzip it. Run the little program. A window with a large rotating spiral will open. Stare at it for a minute - the longer the better. Don't sit too close. Your eyes/brain will start to see details - patterns, 3D-effects, fleeting stroboscopic images - but these are not the illusion!
The real fun begins when you look away.
Do try it - it's well worth it!