Water.

　　2. What's the difference between a hill and a pill？

　　A hill is hard to get up and a pill is hard to get down.

　　3. Why is it useless to send a letter to Washington？

　　Because he's dead.

　　4. A doctor and a lawyer loved the same girl. The lawyer went away for a week and gave the girl seven apples before he left. Why？

　　Because an apple a day keeps the doctor away.

　　5. What will you do if a man-eating tiger is running after you？

　　Nothing. Because I'm a woman.

　　6. What always travels on foot？

　　A shoe.

　　7. Where can happiness always be found？

　　In the dictionary.

　　8. What is higher without a head than with a head？

　　A pillow.（枕头）

　　9. Why don't you advertise for your lost dog？

　　He can't read.

　　10. On which side does a bird have the most feathers？

　　The outside.