The first illusion which I can remember captivating my attention was one I first saw around 1967 whilst stationed at Glasgow AFB in Montana.  Not only was the drawing fascinating to look at, but the person who did it invented names to go with these strange creations! A Trichotometric Indicator Support goes into a Rectabular Excursion Bracket, and is secured with three Ambihelical Hexnuts. Fascinating!

This looks like the washer which would be needed with the ambihelical hexnut.

Recently I ran across this challenge on a Facebook post: How many squares do you see in this image? This isn't an optical illusion, but it will challenge you optically. It took a while, but I came up with 40.  Click here to see all of them.

Are you looking downwards into a three-sided object with a small cube in the rear corner, or are you looking upwards at the bottom of a rectangular block with a square recess in a front corner? Or, maybe, you are looking down on a tall, square object shaped like a skyscraper and a square cube is missing from the top!

This illustration seems to deny common sense: By rearanging the shapes, there is a white space in the bottom arrangement, and it is not in the top arrangement.

You went to the lumber yard and bought 10 boards.  Well, that's what you thought you bought, but when you got home you had only 7!  Obviously, you have trouble counting.

Some people are really great at modifying fonts to help illustrate a message other than with just the words themselves. In this example, a mirror creates a second word by inverting the letters of 'teach' to create the associated outcome of 'learn'. Simply awesome!

If you are familiar with tongue and groove flooring, you'll certainly appreciate the difficulty you'd have trying to install this piece!

Triangles are frequent subjects of optical illusions.  Here are four examples.

Is the inferred white triangle more dominant than the one behind it outlined in black? This is one case where you can definitely see something that isn't really there.