Answer:
Temporary evidence of where you've been strolling
What is filled with air and sometimes a gas, tied to a string and often seen on your birthday?
What is it that you ought to keep after you have given it to someone else?
What is something you will never see again?
Without this, H2O would just be 2O
Two little holes in the side of a hill. Just as you come to the cherry-red mill.