The kilogram

Here's the link to the wikipedia entry for the kilogram. There's a lot of info here, some of which you won't understand, but a lot of which you will. And it should fascinate you to know that there still is so much work going on to define something as seemingly simple as a standard of mass.

Here's a story about the kilogram losing mass.

Who knows, maybe you will grow up to be the scientist who develops the most precise mass standard for the kilogram. That would be one major way to change the world.