How much you weigh depends on where you are. At the moment,
you are about 6,500 kilometers from the center of Earth. Your
distance from the center of Earth determines what you weigh right
now. Your weight depends on how strongly Earth pulls your body
toward its center.
Earth's pull is a force called gravity. Gravity is a force of attraction
between two masses. The two masses could be Earth and you.
In fact, the two masses could be any two objects in the universe.
All objects in the universe are affected by gravity.
The more mass two objects have, the stronger the gravity is between
them. Picture a person who is twice your size. That person has twice
as much mass as you. Therefore, the force of gravity between Earth
and that person is twice as strong as the force of gravity between
Earth and you. Earth's gravity pulls that person to its center more
strongly than it pulls you. As a result, that person not only has more
mass but also weighs more than you do.
Now imagine taking a trip to the moon. After all, haven't you always
wanted to go there? The moon has much less mass than Earth. As a
result, the force of gravity on the moon is not as strong as it is on
Earth. Because the moon's gravity would not pull you with as much
force, you would weigh less. In fact, you would weigh only about
one-sixth of what you weigh on Earth. But you'd have the same
amount of mass.