EXPERIMENT
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Show the class a jar of water, and explain that most water has some air mixed into it. Shake the jar vigorously and have them look at the bubbles of air as they float to the top. Explain that, although most of the air bubbles disappear, many tiny ones remain in the water.
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Explain that, because it is too hard to see air in the water, you will use some colouring for this experiment. Add several drops of food colouring to the water.
< Line a board with a white paper towel and slowly pour the coloured water across the towel. Ask students to describe what happens to the colouring. Some colour passes along with the water, and some stays behind in the towel.
< Ask students to imagine that the colour in the water is air and that the towel is a fish’s gills. Have them suggest how fish might get air from water. Fish move water through their gills and absorb air from the water as it passes along the gills. They take a mouthful of water, close their mouth, then push the water out through the gills in their throat. The gills are made up of rows of very fine folds of tissue similar to skin. The salmon absorb the air through these special gill cells.