lesson
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Dolphins swim in the ocean, but they are not fish. So what makes a real fish?
To be a true fish, an animal must share a special set of body features built for life underwater.
Let's check out the four big clues scientists use to spot a fish.
The Four Fish Clues
First, fish live in water their entire lives, from fresh rivers to deep salty oceans.
Second, they breathe underwater using gills, which are feathery body parts that pull oxygen straight out of the water.
Third, fish swim using fins to steer and push forward instead of legs or paws.
Fourth, most fish have bodies covered in small, overlapping plates called scales with a thin slimy layer for protection.
📊An interactive, colorful diagram of a friendly cartoon fish swimming in blue water. Four callout labels with clear arrow pointers highlight key fish parts: 1. 'Gills' (curved flaps behind the eye with animated gentle blue bubbles showing water passing through to extract oxygen), 2. 'Fins' (highlighting dorsal, tail/caudal, and pectoral fins with gentle swimming wiggle animation), 3. 'Scales' (a zoomed circle showing shiny overlapping fish scales), and 4. 'Water Habitat' (underwater background with gentle seaweed and bubbles). Clean white card UI on soft light blue background (#f0f8ff), rounded 12px corners, large readable text for 4th graders.
How do gills actually breathe without air? Let's take a closer look.
How Gills Work
A fish gulps water through its mouth and pushes it out across the gills on the sides of its head.
Tiny blood vessels inside the gills grab the invisible oxygen trapped in the water, just like your lungs grab oxygen from the air.