1) determine if 3 lengths are sufficient to form a triangle
2) Given 2 lengths, determine the range of lengths for the 3rd side
Take a piece of spaghetti and break it into 3 pieces and set 1 of them aside (keep the other 2 handy, we will call them sides 1 and 2). Now take another piece of full length spaghetti (we'll call it side 3) along with sides 1 and 2 AND make a triangle.
Did you notice a problem?
demo of the problem
Source: Steve Johnson 11-20-12
Now lets talk about why you could not make a triangle
Source: Steve Johnson