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There are some key words that you need to know for circles: radius, circumference, diameter, sector, segment, tangent, chord and arc. In this video you will discover what they all mean.

This video discusses the topic of circles.

These two short online tutorials discuss how the earth rotates and then the earth's revolutions.

In the first URL, the YouTube video explains why earth has spring, summer, autumn and winter seasons. The second URL summarizes the key takeaway points.

This video explains the equation of a circle.

This 32 minute long YouTube video investigates the water contamination problems in Tanzania, with specific focus on Dar Es Salaam. 

This 11 minute long YouTube video describes the various causes and types of water pollutants.

This YouTube video investigates what water pollution is and some examples and effects of water pollution.