I find myself lying to my students. A lot. I suppose it doesn't much bother me on a moral level. For one thing, my conscience is perhaps less muscular than it ought to be. For another, I'm generally pretty open with my kids. They know, for instance, that I'm divorced. That I'm quitting smoking for my 30th birthday. That for several years I was professionally violent. I go out of my way to let them know that, within reason, I won't shy away from their curiosity. Still, I lie.

Some of the lies are self-serving. (*I will *totally* have this graded by tomorrow.*) Some are self-preserving (*Tell your mom it must have ended up in my spam folder.*) Some of them aren't really even **told**, more like **nurtured** from the soil of the teenage imagination. (*I have served time in prison. I am the father of the health teacher's new baby.*) But the lies that keep me up at night are mathematical. (*You can't take the square root of a negative number. Functions approach their asymptotes, but never touch them.*) And, in this last category, I'm not sure what to do about it.

A while back I wrote a post on the equals sign in geometry. One of the difficult concepts for new geometry students is **congruence**, which looks an awful lot like equality, but isn't quite. Equality can either be used a relation between numbers, or between geometric objects that are identical as sets. Objects with the same fundamental characteristics (e.g. polygons with the same corresponding side lengths and angle measures), on the other hand, are said to be congruent. The standard high-school-level method for keeping the two concepts straight is to say something like, "Numbers are equal, geometric objects are congruent." Full stop.

But this is a lie. Things are more complicated than that. In fact, congruence is a weaker condition than equality (geometric equality implies congruence, but the converse is not true). So this equality/congruence lie was one I was just bound and determined not to tell. Of course (and many of you can see this coming), things went disastrously. A few kids internalized the message, but most were so paralyzed by the nuance that they fell back on a self-constructed numbers:equal, objects:congruent shortcut, which is true in all but the pickiest of circumstances.

Christopher Danielson tried to warn me:

"What does this expanded version of "equal to" ... get you that this simpler distinction doesn't?"

The answer, of course, is nothing. It gets me nothing, really, except the satisfaction of not having heaped one more lie onto the steaming pile of lies that another teacher, in a faraway time and place, is going to have to un-lie. But at what cost?

So this is my struggle. How do I determine the absolutely necessary lies from the lazy ones? How do I lie with the bare minimum frequency and intensity? My kids are certainly to the point where I can un-lie the domain of the square root function, but they certainly weren't when their first teachers fibbed it with such good intentions. Most of my kids will never obtain the skill set to get at a mathematically rigorous definition of an asymptote, so I'm stuck with: *a line that a function approaches, but does not reach, except in certain situations where a function can indeed cross the line, so long as it behaves like an asymptote at some point...eventually*. But for a few of them, this is another lie to be untold as they move toward calculus.

I have no idea how to **avoid** lying, but I'm going to try to at least be more **explicit** about lying. I'm going to try to help kids mentally bookmark my pretty little lies in the hopes that the reptilian parts of their brains will expect to be compensated with genuine truth at some point...eventually.

JJ Burnam (@AmbivalentGeek)It's less a lie to be untold than a truth to be discovered. I'm glad you're worried about this.

Chris LustoI like that. It's a rationalization, but I like it. In fact, maybe that's

whyI like it. When I'm feeling particularly guilty about some mathematical untruth, I will close my eyes and dream of future discovery.dweesGreat observation. I know that in my own career as a mathematics teacher, I've told many similar lies. I wonder, could we create a book out of these?

Chris LustoI'd read that, actually. It'd be an interesting exercise to try and put them all in one place, both as a teacher and as a student.

Mike MohrI found myself in a similar conundrum this year when I was teaching my students in Alg I and actually today. We were discussing how many solutions one would get out of the Quadratic Formula when the square root (b^2-4ac) was negative. I idiotically mentioned the concept that when this is a negative number you do not get "no solutions" but rather do not get "real" numbers. One person in my class went oh imaginary numbers while the other 29 not only were utterly confused by this idea of an imaginary number. This caused them to lose confidence in the first steps of the quadratic formula. So, I agree, these lies are needed but it is a fine line we have to walk.

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LSquaredA very cool question. Much trickier than it looks!

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David WeesI recently used this blog post in a workshop, and I thought I would share some of the comments educators wrote about it:

- I understand the struggle - how do we teach students concepts they are ready for + hold back others they are not ready for until later? I like to use a phase, "For now, this is the case, later you will learn other cases."

- I appreciate your reflection. Keep teaching the truth and making it explicit that in this part of the math story, we know "this" is true, and you'll learn more in the next chapter.

- Great point! I learned the hazard of "lying" (which I've heard called "rules that expire") when I moved from 1st to 4th grade. They called me on several "untruths". I now try never to make a statement which isn't always true.

- Kids need to be taught at a more simple level then they can grasp higher concepts. Where is the line between a lie and a simpler explanation?

- And how do we undo the lies of those before us? Students learn multiplication as repeated addition, but that's not really what multiplication is. How do we best let students know they've been lied to?

- I often tell my students that you "can't" subtract a larger number from a smaller number. I knowingly lie about that should clarify that it's not that they can't, but just that they don't know that kind of math yet.

- I have found that giving general rules help - but I also let students know that they will find situations later - once they have more skills to be able to do things they currently can't.

- @MeaganRhodes - Your post made me laugh and think at the same time. I imagine those students who go on to learn beyond the lies will do so with a gleeful sense of discovery.

- Agree that there probably isn't anyway to completely avoid lying. Keep making professional decisions of what your students can "handle" and choose your lies accordingly.

- Lying is that all do! Education people know this. Our job is to learn to be a discerning, consumer of "facts".

- Though about lying, you are sharing a lot of honesty here.

- Don't lie, state conditions.