Why are there white streaks in the sky?

First of all, are you a child asking this simple question?

If not, and you are an adult, maybe you were sleeping in elementary class at the back row =)

If you are, I will gladly answer your question.

Airplanes usually have jet engines that produce hot air as by-products of burning jet fuel. The jet propulsion system pushing the airplane forward leaves a trail of hot air. When hot air comes in contact with cold air, it produces air droplets. These droplets is what we see as white streaks in the sky trailing an airplane...

An example is when you blow your breath to the cold winter air and you can see steam...