Try asking your class:
"What do I need to make a triangle?"
They'll probably say "three sides!"
"Will any three sides make a triangle then?"
They'll probably say "of course!"
Well, this investigation gives them a chance to play around with this idea. They will quickly realize that sometimes just putting three line segments together doesn't make a triangle. It depends on the lengths of those segments.
Common Core Math 7 has that sneaky little standard that hints at the Triangle Inequality Theorem (7.G.2) which basically answer the question, "If we're given a bunch of line segments, can they form a triangle?"
Rather than telling them a rule or set of rules, why not have them have them make sense of it themselves first? Just prepare the investigation and let them work!
Let me know how it goes!