I have a challenging class this year and with that I mean many behavioral issues. My students were having trouble because they were saying mean things to each other. I did this toothpaste activity to show a concrete example of the affect words can have on others. We talked about how our words come out and you can't get take them back once you've said them. No matter how hard you try, you can't get EVERY piece of toothpaste back inside the tube...just like the mean words we say. I think this really hit home for most of them because so far so good!
I think they got the point when one of my students said "Ugh, it is just not possible...we will be doing this forever!"