Good Guys and Bad Guys:
I used this little trick with my 4th graders this year, and they LOVE it! I learned it from a teacher friend, but I don't know exactly where she found it.
It helps with Rounding. For example, if they are asked to round 345,762,138 to the nearest ten thousand, they would do the following steps: First, find the rounding place and underline that digit (the 6). Next, they look next door at the 2, and draw a box or door around that number. Then they ask themselves is the 2 a "Good Guy" or "Bad Guy." If the number is a 0,1,2,3,4 it is a "Good Guy" if it is a 5,6,7,8,9 it is a "Bad Guy." The 2 is a "Good Guy" and he says, "6, I like you JUST the way you are, don't change, STAY just the way you are." So 345,762,138 rounds to 345,760,000.
If we were rounding to the nearest hundred thousand, they would underline the 7, look next door and put a box around the 6. Then ask themselves if 6 is a "Good Guy" or "Bad Guy." It is a "Bad Guy," so it pushes and pushes on the 7 until it bumps up one to an 8. So 345,762,138 rounds to 345,800,000.
My 4th graders LOVE rounding and using Good Guys/Bad Guys. I made two posters to help remind them of the steps. Here are some pictures of those anchor charts. Enjoy, and feel free to use this trick with rounding in your class. =)