This is an activity that takes 1 - 2 class sessions and teaches your young students (grades 2 - 5) something that is less scary than human reproduction or racial discrimination: negative numbers!
Seriously, if you aren't introducing your students to negative numbers at an early age, then you're not doing the best job you could at being a math teacher, and I'm not saying that to hurt your feelings, but because I want you to look good (and, as my hero, Vidal Sassoon said over and over again, "if you don't look good, I don't look good."
There are many reasons you should be teaching your 2nd - 5th graders about negative numbers:
They already know about negative numbers. If you live anywhere in the world where the temperature goes below 32 degrees, your students already know that this “below freezing.” They might also live somewhere (or have heard about) places where the temperature is so cold that it goes “below zero,” or “minus ____ degrees.” If your students know that babies are not brought by storks, then they probably already know about negative numbers. So let’s just teach it anyway.
Negative numbers help students to understand commutativity. One of the most important concepts that students need to know about subtraction is that it is not commutative. It’s fine to show that 9 - 5 ≠ 5 - 9, but what is even more important is to show that both are equally plausible and that both have solutions. I really don’t like it when teachers (or anyone else) says “you can’t do 5 - 9,” because that’s a lie. You can do 5 - 9, it’s just that it comes out to be a different kind of number. In the same way that we lie to our students when we say “4 doesn’t go into 10” (it does, but not evenly) or 8 doesn’t go into 4 (it does, but the solution is less than 1), we are not telling our students the “full truth” about how numbers work when we say “you can’t take (insert a random number here) from (insert a smaller number here.)
Negative numbers open up discussions about the history of mathematics: One of the things that it is important for students to understand is that negative numbers did not come out of nowhere, and that it has changed over the last 2,000 years, including the recognition and use of negative numbers. In fact, there was a time when the existance of negative numbers was in dispute (mostly by stubborn Europeans; other cultures seemed to have no problem adapting to them.)
Negative numbers allow students to use alternative algorithms that make sense and are consistent with their understanding of how numbers work. A student who understands the basics of calculating “small minus big” now has the capacity to try some really cool algorithms that are a lot “neater” and interesting than those who only know “subtraction is always big minus small.”
Knowlege and comfort with negative numbers will ensure success in later grades. Let’s face it, negative numbers are not like sexual preferences or past experience with drugs: you can talk about them without embarassing yourself too badly, especially if you have a context that is provocative and resonates with your students. The teachers your students will have in the future will be okay with it so long as you’re teaching negative numbers in a “positive” way (if you’ll pardon the pun....)
So get with it: teach your students about negative numbers and they’ll be all the better mathematicians for it. Go for it!
Comes with complete teaching instructions!