Matthew 16:2b-3 talks about a time when some people came to Jesus and asked him to show them a sign from heaven to prove that he was really who he said he was. But Jesus told them that they could tell the weather by looking at the sky, and they should be able to understand what was happening in the world around them without needing a special sign from heaven. He was trying to teach them to be aware of their surroundings and use their own knowledge and observations to understand the world.