Time is a mystery. And even though we can't see it or touch it, we can measure it--with clocks. But a long time ago, there were no clocks. Back then, people measured time by the sun. Egyptians were the first to keep track of time, with a shadow stick clock, and they were the first to divide time into hours.

While clocks kept track of hours, the moon helped keep track of months. The first calendar was a lunar one, but it was soon discovered that it was not exact. The calendar was then based on the sun, and it was Julius Caesar who added a day to February every fourth year to make the calendar even more accurate. Robert E. Wells covers the history of measuring time with funny and interesting examples and illustrations....

If you want to win races, never race a cheetah--no animal on earth can run faster! A peregrine falcon can swoop faster than a cheetah can run, but that can't even compare to an airplane, a rocket, or the speed of light. Full color....

Introduces the concepts of very large numbers, up to a googol, and multiples of ten....

THIS EDITION IS INTENDED FOR USE IN SCHOOLS AND LIBRARIES ONLY. Provides a simple introduction to the use of levers, pulleys, and wheels to move heavy objects....

Move to another planet? Sounds interesting! In our imaginary spaceship, let's check out the planets in our solar system. Mercury is closest, but it has no air, and it's either sizzling hot or bitterly cold. The atmosphere on Venus is poisonous; plus, human beings would cook there. Mars might work, but you'd always have to be in a protective shelter. And if you got to the outer planets, you couldn't even land as they are mostly made of gas! Our home planet is looking good. Why is Earth so comfortable for plants, animals, and people? As Robert E. Wells explains, it's because of our just-right position form the sun, marvelous atmosphere, and abundant water. Our planet is very special and perfect for us, and that's why we must do all we can to keep Earth healthy....

In the Arctic, the summer ice is melting, making it hard for polar bears and their cubs to survive. Why is the world getting warmer?

The heat of the sun is trapped by the _greenhouse_ gases that surround Earth_carbon dioxide, methane, nitrous oxide, and water vapor. If there is just the right amount of these trapped gases, the air is warm enough for plants, animals, and people to thrive. But now there is too much greenhouse gas, especially carbon dioxide. Polar bears, and all of us, are in trouble.
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A pygmy shrew is small; it's among the smallest of mammals. What could be smaller? A ladybug? A protozoa? Yes, there are smaller things than a pygmy shrew, and in this delightful book, Wells introduces readers to all of them--right down to the atoms and quarks!...

It's hard to imagine-the molecules in the glass of water you just finished might have once been part of a water hole that dinosaurs drank from! In this fascinating new book about the water cycle, readers will find that while it might be hard to imagine, it's true.

The author, Robert E. Wells, explains the complete water cycle and also discusses ocean currents, ocean and lake habitats, and hydroelectricity. He also touches on water pollution and our responsibility to keep our water clean. The author's bright cartoon illustrations make this information especially appealing to kids....