should be asked, as part of a strategy for building the expectation that conjectures and their proofs are part of the math game.One group of third graders we know love a set of logic puzzles about two families: the Liars (who can never tell the truth, not even by accident!) and the Truthtellers (who can’t lie, even in error).5 For example, “You meet Dale and Dana. Dale says ‘We’re both Liars.’ What family is Dale from? What about Dana?” Playing with puzzles like this gives students experience with looking for contradictions (could Dale be telling the truth?), tracking logical consequences, and (sometimes) trying all the cases (being systematic enough to know what all the cases are). Many students find it positively funny to consider what to make out of the statement “I am a Liar.”Here’s another example. Fifth grader Naomi proudly proclaims that she has discovered a new math rule: that whenever the perimeter of a rectangle increases, its area also increases. She uses a table to demonstrate her rule: Rectangle 1 Rectangle 2 Rectangle 3 Rectangle 4length 4 units 6 units 8 units 10 unitswidth 2 units 3 units 4 units 5 unitsperimeter 12 units 18 units 24 units 30 unitsarea 8 square units 18 square units 32 square units 50 square unitsHer rule seems to make sense; it looks like it might be true. Students might start by trying to find other rectangles to show that the rule is true. They are trying to prove her rule by showing lots of examples that support her claim. But, if they are systematic and planful in their choice of examples (or even by mistake), they are likely to find rectangles that contradict Naomi’s rule. A proof by counterexample, a truly powerful kind of justification! This need not even be a defeat for Naomi. It is harder, but possible, to ask under what conditions her rule is true! This might lead students to think about shape, and is a good foreshadowing of ideas of geometric similarity.As long as the patterns are interesting enough, elementary school kids love finding them and explaining why things are the way they are. And, when the proofs and proof styles are developmentally appropriate, the kids can often learn to do them very well. The curiosity to know what and why, and the skill to find out, are important to foster and develop in kids, not just as a precursor to proof in high school geometry, but because these traits are valuable outside of mathematics as well as within. Whether one becomes an investigative journalist, a mechanic (auto diagnostician), a doctor (human diagnostician), or a scientist, these same inclinations and skills are essential. Kids of 9 and 10 playing four-square at recess may spend more of their time debating the rules, or whether a play met the current rules or not, than in the actual play. They positively thrive on looking for and assessing criteria and on proving things right or wrong.5 Adam Case’s Who Tells the Truth? has a nice set of these Liar/Truthteller puzzles. This kind of puzzle, famous in mathematics, is found in many recreational math books.