Three sticks of 1 cm, 2 cm, and 10 cm cannot make a triangle — no matter how you spread the two short sticks, they cannot reach the long one. Change the three side lengths and explore the condition yourself.
Why is this condition necessary? To get from one end of a triangle to the other along its boundary, you must travel along the other two sides. That route cannot be shorter than going straight between the two points, because a straight line is the shortest path. So the sum of any two sides must always be greater than the remaining side.
If the sum of two sides is exactly equal to the remaining side, the result is not a triangle but a completely flattened straight line — that does not count as a triangle.