Putting the Jelly in Jellyfish

Putting the Jelly in Jellyfish

November 8, 2019

We have jelly, and we have fish…and then we have jellyfish. Jellyfish can be smaller than your fingernail, or up to 6 feet wide! They can weigh as much as 440 pounds, but not all of them are dangerous. Even so, our fan Adrian N. just had to ask, how many jellyfish are there in the world? (and drew this great picture, too!) Turns out it’s hard to swim around counting these things. Scientists say jellyfish make up 40% of the weight of all living things in the ocean, which comes to about 3.4 billion tons, or almost 7 trillion pounds of goopy jellyfish.  If jellyfish weigh 4.4 pounds on average, that gives us between 1 to 2 trillion jellyfish. Too bad they can’t be part of our PB&J sandwiches.

Wee ones: Some jellyfish are flat circles. Find 3 circle shapes in your room.

Little kids: Which weighs LESS, a 5-pound jellyfish or an 9-pound jellyfish?  Bonus: How much would a jellyfish weigh if it were halfway between those two?

Big kids: If you go fishing and pull in a thousand 6-pound jellyfish, how many tons do they weigh? (Reminder if needed: A ton equals 2,000 pounds.)  Bonus: A lion’s mane jellyfish’s tentacles (squiggly legs) can be 120 feet long! If your house is a perfect square, with same length as width, how wide could it be for the jellyfish still to be able to wrap around once?




Wee ones: Things might include plates, the rims of cups, and the faces of clocks and watches.

Little kids: The 5-pound jellyfish weighs less.  Bonus: 7 pounds.

Big kids: 3 tons, since they weigh 6,000 pounds.  Bonus: 30 feet wide (120 split into 4 equal parts for the 4 sides).

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About the Author

Laura Overdeck

Laura Overdeck

Laura Bilodeau Overdeck is founder and president of Bedtime Math Foundation. Her goal is to make math as playful for kids as it was for her when she was a child. Her mom had Laura baking before she could walk, and her dad had her using power tools at a very unsafe age, measuring lengths, widths and angles in the process. Armed with this early love of numbers, Laura went on to get a BA in astrophysics from Princeton University, and an MBA from the Wharton School of Business; she continues to star-gaze today. Laura’s other interests include her three lively children, chocolate, extreme vehicles, and Lego Mindstorms.

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