The witches prophesy foretells that macbeth will be safe until birnam wood comes to dunsinane which seems impossible, but comes true. how?

In Shakespeare's Macbeth, the witches show Macbeth an apparition which tells him that he will not be defeated until Birnam Wood marches to Dunsinane. Macbeth, upon hearing this, feels relieved as he understands that it is impossible for the trees in the woods to march from one place to another. However, when Macduff and his army are coming to Dunsinane to fight Macbeth, they cut off branches from trees to hide behind them as they march towards Macbeth's castle. Because the army was hiding behind the branches, it looked like the trees were marching from Birnam to Dunsinane. In this manner, the prophecy was fulfilled.