Dangerous Mix joke

A female student wished to make some potassium hydroxide solution (aqueous) and decided to throw a large lump of potassium into a bucket of water.

Her professor observed what she was about to do, out of the corner of his eye and hurried towards her, and after confirming this was what she was intending to do, asked her first to stir the water in the bucket for five minutes before adding the potassium.

She was puzzled and ran after him to ask the purpose of this action.

"It will give me time to get away" said the professor.