Impossible trident is one of the most misterious impossible figures, and it holds special position among them.
If you obscure top part of the trident, you will see quite real scene - three rounded columns. Then, if we obscure bottop part of the trident, we'll see possible object again - a block with two pillars with sqare section. But if we observe full figure, we'll see that two square pillars turns into three rounded columns.
So, you can see that foreground of the impossible trident conflicts with background. The middle column which is in the background at the top turns into foreground at the bottom of the trident. Besides changing foreground with background, you can see also that flat faces of pillars at the top turns into rounded at the bottom.
Our brain analizes the shape of the trident and tries to count pillars. We see two pillars at the top and three at the bottom and understand that this object is impossible. If count of columns would be more (for example 7 or 8) the paradox was less obvious.
It was claimed in some sources that impossible trident belongs to impossible figures which can never be realized in the real world. This is not the case. All impossible figures can be created in the real world, but with restrictions. All impossible figures look impossible from a single point of view. You can read more about creating impossible figures in the real world in the article "Impossible figures in the real world".
Nobody knows who discovered impossible trident. It appeared in various sources in the middle if the 60s of he last century. The well-known illustration by Norman Mingo was printed in the "MAD" magazine in march of 1965.
Many artists used impossible trident in art. Japan artist Shigeo Fukuda created impossible columnade in 1985, Inspired by the impossible trident, english psychologist Roger Shepard from Stenford created image of impossible elephant.
Impossible triangle has many names:
"Poiuyt" (just look at the top letters row of your keyboard)