两个英语谜语,I am a word .i have three letters .my first letter is

两个英语谜语,
I am a word .i have three letters .my first letter is in pig but not in big .my second letter is in bed and in pet.my third letter is not in let but it's in net.what am
two fathers and two sons buy three caps,and then everyone has a cap.why?
我上面的是两个谜语,有三行,第一行、第二行是一个,第三行是一个.