The Gregorian calendar

Developed nearly 500 years ago, it is a variant of the Julian calendar. Today it is the most widely used calendar.

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This lesson presents deserts and their flora and fauna.

Why is seawater salty?

Water covers about 71% of the Earth’s surface. But why are there two types of water on Earth,...

Icebergs

Icebergs are blocks of frozen freshwater floating in the sea.

Glaciation

The last Ice Age ended about 13 thousand years ago.

Weather fronts

When masses of cold and warm air meet, weather fronts develop.

Karst region (intermediate)

Karst formations include dolines and dripstones.

Underground waters

Groundwater and aquifers are types of underground waters.

Greenhouse effect

Human activity increases the greenhouse effect and leads to global warming.

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