Good Riddles

Question: Why was the fat man sad?

Question: If I am buying 9 for $1, 18 for $2, 125 for $3, and 1222 for $4, then what am I buying?

Question: what is a cow moving backward?

Question: Why was the mother firefly unhappy?

Question: Travel a mile and I will change.
Travel a million and I will end as I started.

What am I?