When you’re little, growing taller and getting bigger is a… well… BIG deal! Children often look forward to certain milestones that they’ll reach only when they’re tall enough or weigh enough. Of course, when you get older, getting bigger isn’t nearly as fun anymore.

Have you ever stopped to wonder exactly what weight is? To learn about weight, we must also take a look at a related idea: mass.

Mass measures the number of atoms in an object, combined with the density of those atoms. Mass is how much “stuff” is in an object.

Mass isn’t the same as size, though. For example, a helium balloon is much bigger than a lead bullet, but it’s also lighter and less massive.

Weight, on the other hand, measures the pull of gravity on an object. On Earth, mass and weight are closely related. More massive objects tend to weigh more.

Weight changes if the amount of gravity pulling on an object changes. Take yourself, for example. If you weigh 100 pounds on Earth, you would weigh only 16.6 pounds on the moon! That’s because the moon’s gravitational pull is significantly weaker than Earth’s.

If you went to the moon, you wouldn’t have any less “stuff” in you. Your mass would remain the same. Your weight, however, depends on gravity and would be significantly smaller on the moon.

Have you ever noticed that the abbreviation for pound is “lb”? What’s the deal with that? Wouldn’t it make more sense if it were “po” or “pd”?

For the answer, we have to travel back to ancient Rome. The ancient Romans gave the name “Libra” to a constellation they thought looked like a pair of scales. The astrological sign Libra took its name from this constellation.

When the ancient Romans referred to weight, they used the term libra pondo. Libra meant “weight” or “balance scales,” and pondo meant “pound.” They eventually shortened the phrase to just libra, which they abbreviated “lb.”

Oddly enough, our English word “pound” comes from pondo, but we kept the “lb” abbreviation of libra for “pound.” Weird, right?

There’s just no rhyme or reason as to why the word “pound” was taken from one Latin word but its abbreviation was taken from a different Latin word. It’s just one of the mysteries of language!


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  1. I loved the e-mail abbreviations. It was fun guessing them b4 I went ahead and clicked on the answer.

    The wonder today was one of the really good ones. It was something that I had never thought about. :)

    • Thanks for letting us know how much you liked today’s Wonder, Grace! That email game WAS a lot of fun…we had fun playing it, too! :-)

    • We really appreciate you letting us know the value you and your home students find in Wonderopolis! We appreciate you, Wonder Friend! :-)

  2. The dogs in the video were very cute! Our principal has a dog that looks like Javi so we loved making that connection to the video. The information was good too, we never thought about how “lb” related. Thanks for the great info!!

    • Thanks for being AWESOME Wonder Friends, Kerrick Elementary 2nd/EBD classroom! We thought the dogs were super cute, too! We wish we could give them a hug and a few doggie treats (although that might not help with their reduction of “lbs”)! :-)

  3. LOVE this wonder for so many reasons. First Grace hit on it what a great game to help the older generation with the e-mail abbreviations. As a teacher I love the connection with Ancient Roman times, I will be able to use this wonder with math and word study-you taught me something new today. THANKS!!!

    • Well, then we’re glad you liked this Wonder, Jusin! We like it when our Wonder Friends tell us all the great stuff they learn each day! :-)

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Have you ever wondered…

  • Why is “lb” the abbreviation for pound?
  • How are mass and weight different?
  • How did the constellation Libra get its name?

Wonder Gallery

Try It Out

Think you know how much some common household items weigh? Get a friend to help you in the kitchen and put your weight-guessing skills to the test!

Have your friend grab a few items from the cupboards to use as test objects. For example, bags of sugar or flour work well.

With your eyes closed, have your friend hand you items randomly. Your job? Try to guess how much they weigh! With a little practice, your guesses will likely improve.

If you want to test objects to make sure their stated weights are correct, grab the bathroom scales and take some measurements to confirm the objects’ weights.

If you want to make your own homemade set of balance scales to compare the weights of small objects, follow these simple directions to make scales out of cardboard and a ruler or a make scales out of a plastic hanger and clothespins!


Still Wondering

Check out ReadWriteThink’s E-mail Abbreviations interactive lesson to learn more about the shortcuts and abbreviations that writers use when they write e-mails.


Wonder Categories/Tags



gravity  lb  mass  measure  measurement  ounce  oz  pound  pull  weight 

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