四十个为什么PDF电子书下载

Contents 1

1．What Caused the Tower of Pisa to Lean ？ 1

2．Was America Discovered Before Columbus ？ 5

3．Does the Whole World Have Time Zones ？ 8

4．How Does a Refrigerator Produce Cold ？ 12

5．What Makes anAirplane Stay up ？ 16

6．What Is Radar ？ 20

7．How Did Our System of Counting Begin ？ 24

8．Who Discovered Electricity ？ 28

9．Who Built the First Lighthouse ？ 31

10．Who Discovered the Atom ？ 35

11．Who Invented the Airplane ？ 39

12．How Big Is the Universe ？ 42

13．What Is the Milky Way ？ 45

14．What Are Falling Stars ？ 48

15．Where Does the Wind Come From ？ 52

16．What Is Fog ？ 56

17．What Is Dew？ 60

18．Why Do Snowflakes Have Six Sides ？ 64

19．Why Does Frost Form on Windows ？ 68

20．How Does Water Change to Ice ？ 71

21．What Is a Rainbow？ 75

22．What Is Light ？ 79

23．How Fast Does Sound Travel ？ 82

24．What Is Energy ？ 85

25．What Is Heat ？ 89

26．What Is Friction ？ 93

27．Why Does Iron Get Rusty ？ 96

28．What Is an Alloy ？ 99

29．What Is Petroleum ？ 103

30．Why Do We Get Tired ？ 107

31．What Happens When We Sleep ？ 111

32．What Causes Our Dreams ？ 114

33．Why Do We Laugh ？ 118

34．What Is Cancer ？ 121

35．Can Animals See in Color ？ 125

36．Why Does a Male Bird Have Brighter Colors Than the Female ？ 128

37．How Do Birds Find Their Way Home？ 131

38．How Do Fish Breathe ？ 135

39．Can Any Fish Live out ofWater ？ 138

40．Can a Porpoise Talk ？ 142

Glossary 145