Former indignant first-graders may be glad to know that in my new lesson plan (not for an actual lesson, just for homework, but still!) I have included the following assessment questions:
Children just learning about addition and subtraction are often told that they can't subtract a larger number from a smaller one. Why? Sometimes these children have already learned outside of class about negative numbers. How would you explain, as simply as possible, to these children why their teacher is apparently lying to them?
I'd say:
Did you know numbers never end? They never end; they go on infinitely. Maybe some of you already have heard about "infinity." So, because there is no end to numbers, we need to break down our lessons to certain kinds of numbers at a time. We call each of these kinds of numbers a system.
When we're teaching you the ways you can use numbers, we use the most simple system. Once you master the ways, you will go on to more complicated systems. So, if a teacher tells you that you can't subtract a bigger number from a smaller number, s/he means you can't do it in the system s/he's using. You most certainly can subtract bigger numbers from smaller numbers in some systems, and you'll learn that when you're [whatever grade]. Right now, we're concentrating on learning the main tricks. It's sort of like walking on a trapeze. When you're learning, you do it, with a safety net underneath. Right now, we're using the easy set of numbers, so if you fall off, you fall safely into the net.
Then I'd yell, "AhhhhhhhhhhhhH look out for the elephant!" but that's just me.