Fish spend their whole life in water. But do they actually drink it? Turns out the answer depends entirely on which fish you ask.

In this episode of Why Though?, Dr Matt Agnew dives into what thirst really is, why saltwater and freshwater fish handle water in completely opposite ways, and why sharks are the cleverest rule-breakers in the ocean. Plus a kid in a shark costume makes an appearance. Obviously.

What you'll learn:

• Thirst is your body's alert system... and your body is about 60% water
• Saltwater fish live in super salty water, so water leaks out of their bodies... they drink seawater to survive and their gills filter the extra salt out
• Freshwater fish have the opposite problem... water rushes into their bodies, so they barely drink at all and pee constantly to stay balanced
• Sharks keep a chemical called urea in their bodies that helps water flow in naturally through their skin and gills... no drinking required
• Sharks have been in the ocean for over 400 million years... longer than trees have existed on land

Key Science Ideas:

• Salinity: How salty water is
• Osmosis: Water moving from a less salty area to a more salty area through a membrane like skin or gills
• Homeostasis: Keeping the body's internal conditions just right so everything works properly
• Urea: The chemical sharks use to stay hydrated without drinking seawater
• Gills: A fish's built-in filter system for managing salt and water

Fun Experiment: The Gummy Bear Grow-Off Grab three cups and label them Fresh, Salty, and Super Salty. Add plain water to the first, two teaspoons of salt to the second, and four teaspoons to the third. Drop a gummy bear into each cup, make a prediction, then check back after two hours and again the next morning.

The fresh water gummy swells up the most as water rushes in. The salty one grows a little. The super salty one barely changes at all... the saltier the water outside, the less water moves in. That's osmosis... the exact same process fish use to manage water in their bodies every single day.

Treat the gummies as science props and wash hands after handling.

Why Though? The show for little scientists who love asking big questions. Follow or subscribe so you never miss an episode.

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