Yet again, if we redefine triangle as anything it then becomes possible. In order to truly make everything possible, we would need to define everything as being everything else, which renders language utterly useless. As a result, the above is impractical, so the way to make something impossible is to state that “it is impossible for this particular object-that-I-am-pointing-to-with-my-finger be anything else”.
However, due to the Heisenberg uncertainty principle and quantum physics, the above is not entirely correct. There is a tiny probability that the object will mysteriously morph into something else, or that it will vanish and appear on a distant planet. However, this is not practical either, since the equations show that one would need to wait longer the the lifetime of a universe for that to occur.