Suppose you had a big triangle ABC, maybe meters instead of centimeters,
drawn on the ground. 

1. If you start at A, walk from A to B and then from B to C, 
then you must have walked farther than if you had walked 
directly along the straight line from A to C.

2. If you start at B, walk from B to C then from C to A, 
then you must have walked farther than if you had walked 
directly along the straight line from B to A.

3. If you start at C, walk from C to A and then from A to B, 
then you must have walked farther than if you had walked 
directly along the straight line from C to A.

Why is this?  Becuas the shortest distance between any two
points and must be shorter than any other path.

So let the third stick be x cm long.

Then the sides of the triangle are x, 5, and 9 cm.

The sum of any two must be greater than the third, so we havd the three inequalities

x+5 > 9,    x+9 > 5  and 5+9 > x
  x > 4       x > -4      14 > x

The middle one is obvious.  The other two give us:

              4 < x < 14, so the third side must be more than 4 cm and 
less than 14 cm.

Edwin