The hydrosphere and its parts
The hydrosphere is Earth’s water shell: ocean, seas, rivers, lakes, glaciers, groundwater and water vapour in the air. In nature water occurs in three states: solid (ice), liquid and gas (vapour). About 96.5% of Earth’s water is in the World Ocean, made up of four oceans: the Pacific, Atlantic, Indian and Arctic. Seas are parts of an ocean, gulfs are parts of water reaching into the land, and a strait is a narrow water passage between two landmasses (for example Gibraltar). The ocean floor is uneven like the land: continental shelf (down to about 200 m), continental slope, ocean floor, underwater ridges and deep trenches. Depth is measured with an echo sounder: it sends a sound pulse and measures the time for the echo to return; sound travels in water at about 1500 m/s and, since it goes down and back, depth = speed × time ÷ 2.
River-basin model: cut a cardboard base, glue on a mountain of cork or plasticine, paint the main river and 2–3 tributaries in blue from the mountain downwards and mark the mouth. Then drip a little water on the mountain top and watch it run from the tributaries into the main river. Safety: use scissors with adult supervision, points down; do not spill water; keep glue and paint out of mouths.