How Fish Sink and Float

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Here's how.

Active Time

45 minutes to 1 hour

Total Project Time

45 minutes to 1 hour

Key Concepts

Physics, Buoyancy, Archimedes Principle

Outline of a fish is drawn around a glass bottle that has a rubber tube and balloon inserted through the cap

Introduction

Have you ever thought about why most fish never sink to the bottom of the ocean or float to the water's surface? How is it that they can stay so perfectly buoyant under water? You might be surprised to hear that most bony fish have a special organ to help them with that: a swim bladder. The swim bladder is a thin-walled sac located inside the body of the fish that is usually filled with gas. Besides helping the fish stay buoyant, it can also function as a sound producer and receptor or as an accessory respiratory organ. Wonder how a swim bladder works? Then do this activity to find out!

Credits

Svenja Lohner, PhD, Science Buddies

This activity is not appropriate for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.

Next: Materials

Materials

Small glass bottle
Balloon that fits inside the bottle
Plastic tubing (at least 30 cm or 12 inches)
Big tub or container, waterproof and preferably transparent. It should have enough room so the glass bottle can float in it freely.
Strong and waterproof tape
Water
(Paper) towels

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