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Why Are There Two Tides a Day?
A TED-Ed lesson on ocean tides — from a giant wave that shocked Alexander the Great's army to Newton's discovery that the Moon's gravity drives the tides, creating two bulges (and two high tides) each day.
How it works
- Press play to start the video.
- Videos 5 minutes or longer pause once or twice for a quick check. Get it right and you're on your way. If not, you'll see a friendly explanation and the video simply keeps going. Short videos skip the checks.
- At the end there's a 3-question test. Score 60% or more to complete the lesson, and 80% or more to master it. You can always watch again and try for a higher score.