The next day, the crowd that remained across the sea saw that there had been only one boat there, and that Jesus had not gone along with his disciples in the boat, but only his disciples had left. Other boats came from Tiberias near the place where they had eaten the bread when the Lord gave thanks. When the crowd saw that neither Jesus nor his disciples were there, they themselves got into boats and came to Capernaum looking for Jesus. And when they found him across the sea they said to him, "Rabbi, when did you get here?" Jesus answered them and said, "Amen, amen, I say to you, you are looking for me not because you saw signs but because you ate the loaves and were filled. Do not work for food that perishes but for the food that endures for eternal life, which the Son of Man will give you. For on him the Father, God, has set his seal." So they said to him, "What can we do to accomplish the works of God?" Jesus answered and said to them, "This is the work of God, that you believe in the one he sent." (Jn 6:22-29)
The reaction of the crowds is beautifully narrated by the author. The crowds were puzzled like the disciples and the people also saw the moves of Jesus in detail. They followed him so closely. They knew that Jesus had not gone with the disciples but when they reached Capernaum they found him there already arrived. The author leaves puzzled with this situation and he does not answer. The answer is found only by those who believe in Jesus whom God has sent.
People who ate the bread and the fish were very factual. They accepted those facts that had taken place in front of them. But they did not proceed from that to the message that Jesus wanted to make them realize and be aware of. Jesus was giving them a wilderness experience: bread from heaven and fish for them to eat. He was leading them into a new exodus; a new Passover. The people did not see the signs they only saw the material food that perishes.
The lesson that Jesus taught the people was that they have to accomplish the works of God. They have to see God in life and live Godly life and reveal God to the world. This can be done only by believing in Jesus. Jesus, because he is the one sent by God. Jesus is Godly and that is why he speaks of God and does Godly things. To do the works of God belief in Jesus is essential.