I don't want to prolong this definition, so I'll keep it short. To prolong is to make something last longer or to stretch it out in time.
You see the word long in prolong and it's no trick. If you prolong an argument with a sibling, you make it longer than it naturally would be. Prolong always has to do with time. When you stretch your neck or a piece of cloth to make it appear longer, you are elongating, not prolonging, it.