I know this one at least.
A man was walking along a deserted beach at sunset. As he walked he could see a young boy in the distance, as he drew nearer he noticed that the boy kept bending down, picking something up and throwing it into the water.
Time and again he kept hurling things into the ocean.
As the man approached even closer, he was able to see that the boy was picking up starfish that had been washed up on the beach and, one at a time he was throwing them back into the water.
The man asked the boy what he was doing, the boy replied,"I am throwing these washed up starfish back into the ocean, or else they will die through lack of oxygen. "But", said the man, "You can't possibly save them all, there are thousands on this beach, and this must be happening on hundreds of beaches along the coast. You can't possibly make a difference."
The boy looked down, frowning for a moment; then bent down to pick up another starfish, smiling as he threw it back into the sea. He replied,
"I made a huge difference to that one!"