Every part is important, if something is going to work right.
Every Christian is important too, because they are all part of one body, the body of Christ. Keep the spring handy, where you can find it easily. During class, you will show the ballpoint pen to the children and ask them what they feel is the most important part of the pen. Your will then give the ballpoint pen to some of the children and ask them to write with it, and try to click it open and closed a few times. They will find that is doesn't work very well, because it is missing an insignificant but vital part, the spring.