An I-message is a way to be strong (assertive) without being mean (agressive) when you are angry or upset or disappointed with something another person has done.
The formula for an I-message is as follows:

I feel ____ (say your feeling) when you ____ (describe the action) because ____ (say why the action connects to your feeling)

The “I-message” is different from a “You-message.” In a “You-message,” you attack the other person, make judgments about him or her, sometimes even call the person names.

For example, say the class is picking partners for an activity. You pick John, who wants to work with his best friend instead of with you. He lets you know by pouting and mumbling, “aaw!” You say, “You want to work with him? Fine! You’re too stupid to work with me, anyway!”

Ask the class: How do you think John might feel about being called stupid? Will he be more or less likely to choose you as a partner in the future?
In this situation, what would an “I-message” be? Elicit possible “I-messages” from the students. (For example: “I feel disappointed when you always want to work with your best friend because I never get to spend time with you.”)

I-Messages can also be used to express positive feelings. (For example, “I’m excited that you are coming because we always have so much fun on your visits.”) Encourage your students to share other examples.

Class Discussion: What are your comments about I-messages and You-messages? Can you see using an I-message the next time you feel upset and think of calling somebody a name? Why? Why not?

Source: Excerpted from a longer activity prepared by Amy Martin - see http://teachablemoment.org/elementary/imessages.html

CRE Calendar Usage: 2nd Edition