Since I'm not Catholic some might argue that I am not qualified to answer, but I think I will stick to what I know.
Let's start with something we can all relate to: weight. Weight and mass are similar, they both tell you how much stuff [matter to be technical] you are carrying around everywhere. Specifically, weight tells you that number in reference to something else with a lot of mass - the earth. Mass is just how much stuff you have. So your mass is the same everywhere, but your weight is different on the earth and on the moon, specifically, you weigh one sixth as much on the moon as you do on the earth.
No one needs to diet here.
Luckily we haven't gotten to the really good part about mass. The really good part about mass, and I really like this part, is that there are two different kinds. That's right. There's not just your mass, there's your mass and there's your other mass. Before you get too excited and back onto the diet line of thought, you should know that they are both exactly the same in all cases measured - ever.
The two kinds of mass are called gravitational mass and inertial mass. I realize that this is probably the hardest most people have thought about mass, but bear with me. First gravitational mass: this is how much something is attracted to something else with a lot of gravitational mass. The earth has a big gravitational mass, a penny does not. This one is pretty straightforward and should be totally familiar. By the way, your bathroom scale measures gravitational mass through weight.
The other kind of mass is known as inertial mass. Inertial mass is how much inertia you have. Wasn't that helpful? From a practical point of view, inertia is how much resistance to motion something has. It's hard to make a big train go fast. Obviously, there is the friction problem, but even in a vacuum with all the WD-40
in the world, I probably couldn't make a train move very fast, while a baseball is no problem. What's the difference between the two? Both are moving horizontally so gravity isn't an issue, but one has a giant resistance to motion [inertial mass] and the other doesn't.
As I mentioned earlier, these two kinds of mass are always the same, and no one knows why. If any readers out there have any clever ideas feel free to let me know in a PM and I might just invite you to Stockholm a few years down the road.
It isn't hard to imagine an object that has different inertial mass and gravitational mass. Maybe it moves very easily but has a huge gravitational pull, or is impossible to push around, but floats slowly down to the surface of the earth. The thing is, these sorts of objects haven't been seen yet.
That's mass.
