system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Has there always been the same amount of water on earth?
gpt
No. It is theorized some/most water was brought to Earth by cometary bombardment. Also, some (miniscule) amount of hydrogen is constantly lost from the atmosphere to space. However, it is relatively insignificant to the total amount. 

Water is also sequestered in rocks and ice in varying amounts throughout geologic history.