Because of the varying sizes and compostion of the planets, each planet has a different amount of gravitational pull. A stronger gravitational pull means that objects are being pulled toward the center of the planet with greater force. The end result is that the object weighs more.
(remember... weight is the measure of gravitational pull on an object, mass is the amount of "stuff" an object is made up of - that doesn't change)
The larger, more massive the planet, the more gravitational pull, the more something weighs.
The smaller, less massive the planet, the less gravitational pull, the less something weighs.
To help give students a feel for these differences, I created these:
(I've got a whole set, this is just a representative sample!)
I used this calculator (very cool - have your students play around with it to find their weight on other planets) and an Earth weight of 50 g.
The calculator gave me the following weights:
Mercury: 18.9 g
Venus: 45.3 g
Mars: 18.8 g
Jupiter: 118.2 g
Saturn: 45.8 g
Uranus: 44.4 g
Neptue: 56.2 g
Pluto: 3.3 g
I then created these containers, by taping two cups, filled with an appropriate amount of stuff, together.
It's probably not the best container, but it met these requirements:
1 - Light enough to account for the weight on Pluto when empty.
2 - Opaque - I didn't want students to see through the container. I wanted it to look like they were all the same thing, they just weighed different.
3 - I had them on hand.
I believe I mostly used dried peas/beans for the filling. Jupiter may have a few pennies or other more dense weight thrown in to bulk it up!
If you have a dense material on hand (lead, or something of that sort), you could use film canisters, which would be sturdier than my creation.