This clever trick is based on the great paradox
invented by Paul Curry, a magician from New York City.
In fact, in both triangles their sides aren't perfectly straight; in
the first triangle its sides are slightly concave, while in the second
one they are convex a little bit as shown in Figure 1.
In this way the differences between the triangles' outlines form two
elongated parallelogram spaces along the trianglesí sides. Their total
area is equal to two small square units, and this makes a hole in the
second triangle. This is shown in Figure 2. Note that both
parallelogram spaces are enlarged to show the principle of this trick.