"Startle the fish, or not. Touching him is kind of mean wouldn't you reckon? I think so."
As seen on various Useless Website indices, wiki's and sites.
How it works (SPOILER WARNING!)
The code is very simply. While one could have modeled it in a more complex way, this was not necessary in my opinion.
There exist various variables containing fish states: regular, neutral (not smiling), sad and dead fish.
Based on some randomness the fish moves after it gets clicked (startled) and the clicks are counted.
At various amounts of clicks, the code switches between these states. After 10 clicks, the fish would go into neutral and after 80 clicks and some randomness it would go into sad. At exactly 100 clicks the fish will go into the state dead and the user will see a message that the fish has died.
Curious users might click the dead fish again. If they do this, they discover a goodbye note left by Startle, the fish. They discover that the message ("Startle the fish, or not. Touching him is kind of mean wouldn't you reckon? I think so.") at the beginning merely introduced the fish named Startle, instead of instructing the player/visitor to startle it.
Bonuses: when clicking 'I think so' in the message at the beginning, indicating that you agree touching the fish is mean, you get to the actual good ending of the game. There, clicking 'Play again?' results in you losing that good karma.