[yongjik]

Crudely speaking, topologists consider spaces as if they're made of rubber - a mathematically perfect rubber that can be made infinitely thin or stretch to infinity. So, a circle can be made with an infinitely thin circular rubber ring, and you just pinch three points and stretch, and you get your triangle, in any shape.

But you can't get a straight line - to do that you need scissors to cut one point of a circle (to be precise, remove a single point) - and then you can stretch the remainder to infinity and now you have your line.