This isn’t an illusion, more a special effect, but I just like bubble pictures.  The question mark soap bubble started out as two photos of real bubbles.  In a later post I’ll go into my the ways I use Photoshop to distort and adapt the bubble images.  For how I take the bubble photos, and who else is doing it, see my earlier posts on bubble pictures, by just clicking on Soap Bubble Pictures in Categories, to the right.