Tapping the lock with your fingers or shaking the lock won't do any good, obviously. How ironic that what you need was being held in your left hand. Think, Miss Umbridge, think. Sam seemed to be getting about as blue as Neptune.

And then it hits you - you needed to tap the constellations with your wand. As you tap the stars that make up Leo, the dots you tapped connect and light up a brilliant white. More and more dots lit up as you tap Sagittarius and then Aries. With all the appropriate dots selected, the colour of them changes from white to green.

The lock clicks and the door swings open. Your friend is free to leave the confines of the cage, though being frozen to the bone might make moving out of it on her own quite difficult.