Parents and other people working with beginners in geometry often find that beginners don’t understand any reason to prove their work. Children in particular can become very negative, sad or hostile about the whole idea of proof. And for very good reasons! This is what Lockhart has to say in his “Lament”:
“The art is not in the “truth” but in the explanation, the argument. It is the argument itself which gives the truth its context, and determines what is really being said and meant. Mathematics is the art of explanation. If you deny students the opportunity to engage in this activity— to pose their own problems, make their own conjectures and discoveries, to be wrong, to be creatively frustrated, to have an inspiration, and to cobble together their own explanations and proofs— you deny them mathematics itself. So no, I’m not complaining about the presence of facts and formulas in our mathematics classes, I’m complaining about the lack of mathematics in our mathematics classes.”
This short paragraph is wonderful, because it lists all the activities you can arrange for beginners to help them find beauty and meaning in proofs:
- Pose your own problems and solve them. The captured process of posing and solving problems is an informal proof.
- Explain solutions to your own problems to others. An explanation to another person is an informal proof.
- Engage others in arguments, discussions, “court sessions,” thesis defenses and other contest-oriented, competitive rhetoric experiences. Competition leads to proofs becoming more rigorous as you defend them against the opposition.
- Work at a level where you will often be wrong and creatively frustrated. If you are not sure if you are right, you will want to prove the truth of your solutions to yourself
What do we have in typical geometry courses? Kids work with pretty obvious facts from classic geometry that some long-dead ancients, not kids themselves, created literally thousands of years ago. The proofs are not made for the purpose of solving a problem or convincing someone who thinks you are wrong. The problems have been solved before the course began, and nobody doubts the rightness of the theorems and lemmas presented for “proving.” Actually, the proofs are not MADE at all! Not by the learners. They are received, from the same ancients, and rehearsed.
Yet the very meaning of proof is in the PROCESS of proving. It is said you can’t really appreciate romance, like “Romeo and Juliette,” unless you’ve had some love-related experiences and feelings. You can’t appreciate other people’s proofs unless you created some of your own, from scratch, and went through situations where proving was vital: defending your creations against the opposition, or else using proofs to convince yourself your solution to a tricky problem is not a mistake or a glitch.

Phoenix Wright, a cartoon lawyer famous for his “Objection!” yell, often quoted in our Math Clubs.
So, you need to invite kids into situations where they will be creating their own statements and proving and disproving them. The situations have to provide “natural environments” for proofs and proving. One easy way is to replicate the natural situations existing in our culture. You know how people “defend” their dissertations? That’s because they are attacked! Typically, if you work in math or science research, you will find yourself in somewhat warlike situations where people actually attack your creations. A good model of that situation, for kids, is a debate or rhetoric club. A game of examples/counterexamples works well: some club members become your opponents, attacking your statements and proofs and creating examples and counterexamples that poke holes in them. “Definition Wars” we sometimes play at the Math Clubs is a good example. As a result of being contested and argued, the proof becomes tight and rigorous. The irony is that most people who learn geometry never find themselves in these situations of “scientific attacks and defenses”, so the rigorous prophylactics against attacks is totally lost on them. Never having experienced the need to defend their arguments, they just don’t feel any need for proofs. You can create the need by providing appropriate activities. Make sure you don’t become too hostile, like some conferences I know

Non-Euclidian geometries, by Dirk Laureyssens
Another good source of situations that require proofs is mathematical weirdness. Sometimes our visual intuitions are only “almost right” - and some weird marginal cases escape them. A proof, again, provides prophylactics against such weirdness and counter-intuitive examples. Ironically, again, most geometry students never meet any weirdness, instead doomed to prove things that look and feel (and are) quite obvious and straightforward. Good sources of mathematical weirdness for your kids are paradoxes, and also optical illusions if you talk about geometry specifically. You can also go into non-Euclidian spaces where our intuitions don’t work as well, for example, studying all your theorems on a ball instead of the plane. This way, the proof becomes a daily tool of exploration and problem solving, rather than the spare fifth wheel you only use once in a blue moon, if ever.