Imagine your friend says "I'm going to president one day". In English, we have a number of phrases to say that we think the other person is being totally unrealistic.

As if!
In your dreams!
You wish!
No chance!
Not a chance!
Not likely!
Dream on!
Never in a million years!
You'll be lucky!
Yes, and pigs might fly!

Explanations

• Phrases 1, 2, and 3 not only show disbelief but are also quite dismissive of the person who made the prediction.
• Phrases 4 and 5 are indicating that you think the prediction is totally impossible.
• Phrase 6 is a little more tentative. You concede that there may be a small chance.
• Phrase 7 suggests that you think the other person is just day-dreaming.
• Phrases 9 and 10 are a little sarcastic. In phrase ten you are saying that the prediction is no more likely than pigs suddenly being able to fly.