《四十个为什么》

Contents1

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

Glossary145