Have you ever looked up and seen a big white stripe going across the sky? It’s the same color as the clouds, but it isn’t shaped like them. These stripes are caused by airplanes, specifically, jets!
Jets burn fuel just like cars do. That’s how they run! Also like cars, they have exhaust, which is the gas (or vapor) that is created when an engine burns fuel. There are different chemicals in the exhaust, but one of them is water vapor, like steam.
When the exhaust escapes from a jet engine, it leaves a trail in the sky. Think about what happens to water when it gets very cold. It freezes! It is freezing cold very high in the sky where some jets fly. The stripes they leave behind them are ice crystals and cooled water. The water droplets and ice crystals are very light, so they can actually float in the sky!
The technical name for these stripes is “vapor trail” or “contrail.” They’re made from the same basic stuff as clouds, but instead of being made naturally, they’re made by jets.