In the figure above, drag the vertices around and try to create all 3 possibilities. The title will change to reflect the type of triangle you have created. (You may have to move the mouse slowly to get an angle to be exactly 90°).
* To see why this is so: The internal angles of any triangle always add up to 180°.
If two angles were greater than 90° they would add to more then 180° just by themselves. Therefore this can never happen.
(Prove it to yourself - reshape the triangle above and try to get two angles to be greater than 90°).