Looking For Wrong Answers

I have, in the past asked students for wrong answers rather than right answers to my questions. When I've done it, it's usually because nobody knows the right answer or how to find it. So I ask for wrong answers to start the thinking process. When a student gives a wrong answer I ask why it's wrong and then we try to narrow down the correct answer from there.

I've been reading Dan Meyer's blog for a while now. He's a big supporter of asking for wrong answers but he doesn't just ask for them when nobody knows the answer. He asks for wrong answers all of the time. I tried the technique last week before asking for the correct answer. I was blown away by the quality of wrong answers I was getting.  Here's the graph we looked at:

We had just calculated the slope of the blue to be -1. I added the red line to the graph and before asking what its slope could be I asked for what its slope can't be. I originally expected a lot of ridiculous answers. I was pleasantly surprised.

 Here are some of the responses I received:

• Can't be negative 1, since the slope is different from the blue line (nice segue into slopes of parallel lines)
• Can't be zero since it's not a horizontal line
• Can't be -1000 (a little silly but nailed down that our solution needed to be between -1 and 0)
• Can't be undefined

The best part about these answers is that they were mostly from students who are afraid to answer questions for fear of being wrong. There was no more fear when I was asking for wrong answers. This tool just moved to the top shelf of the toolbox.