A long, long time ago, there was a famous person in China who we call Meng Zi. During that time, China still was divided into many countries.
There was one time that Meng Zi wanted to go to one of those countries, because the king invited Meng Zi to help them to build a canal.
When Meng Zi was walking, he heard, "Help me! Help me!" Then, he found a small fish. The fish was lying in the road. Meng Zi said, "How can I help you?" The small fish asked, "Would you give me a cup of water? I need some water." Meng Zi said, "Sure. I'll give you some water. But I'm on a business now, since the king invited me to build a canal. The canal will pass through here and connect with the sea. When I build it up, you'll be able to go back home by way of the canal. You don't need a cup of water, you will get the whole water of the sea."
The fish was very angry. It said, "Thank you for your kindness. When you construct the canal to here, I'll be a dried fish. But, if you give me some water now, I'll live longer. When you come here again, you won't be able to see me any more."
The story means that you should just help people with what they want now.