Manchester Utd will win the Champions League.
It will be hot next summer.
Barack Obama will change America.
Be careful. If we make a prediction "using our mind", that is thinking about what will happen at some point in the future, we use will. If we make a prediction "using our senses ", that is with our eyes, ears, nose, mouth or by touching, we have to use to be going to:
Look at those clouds! It's going to rain.
Look at that man. He's so drunk he's going to fall.