When you are asleep, sometimes you dream. Dreams are a little like a movie that plays in your head while you sleep. Sometimes dreams are very ordinary, sometimes they are exciting, and sometimes they are scary. Scary dreams are called nightmares. Often things happen in dreams that aren't possible in real life.
A fantasy is something you think up inside your head when you are awake. Fantasies tend to be things you want to do, and they usually aren't possible. Children often have fantasies about having magical powers.
You make a guess when you come to an answer without very much information, or even any information at all. For example, you might guess which hand someone is holding something in, or you might try and guess how many items are in a jar to win a prize.
A hallucination is seeing something that is not there, whether it is strange colors around the edges of things or objects and people that are not there at all. People sometimes get hallucinations when they have a migraine or when they take drugs. A hallucination is not the same as a mirage.
An idea is something you think of inside your head. Often, an idea is something new that you haven't thought of before, and when an idea comes on suddenly it can solve a problem you have been thinking about.
A nightmare is a bad dream. Nightmares can be extremely frightening, and make you wake up scared. You might also use nightmare to describe a particularly bad event, for example "we had a nightmare vacation".
You think when you make words and pictures happen inside your head. You might think about nothing in particular, or you might think about how to solve a problem.
A wish is something you really, really want. Some people wish for possible things, like "I wish I had a better job" but other people might wish for impossible things, like the man in the picture has wished he can fly. Some things are said to be able to grant wishes. When you blow out your birthday candles you can make a wish, and fairies are said to be able to grant wishes.