Take a triangle, ABC, and an arbitrary point, D, in its interior.
How can we prove that AD + DB < AC + CB?

The fact seems obvious, but when the problem is presented on its own, outside of a textbook or some course of study, we have no hint as to what technique to use to prove it.
Construct an equation?
Apply the Pythagorean theorem?

I trust the flooblers to find a formal proof...