 What we have here are three tennis balls. They look the same, they are the same size and color and they behave the same. So we're going to let them represent three isotopes of hydrogen. Since they look the same and chemically behave the same, the only way that they're different is in their mass. So we're going to illustrate that by putting them in water. You see that the level at which they float, this one is lighter, a little heavier, and this one's even heavier. The volumes are the same. Since their densities are different, we know their masses have to be different.