CaC₂ + H₂O + random items from chemistry shelves = grey, gritty foam which rises like cement merangue from beaker and flows over desk and onto floor. Mr Gobel chooses that moment to saunter down the asile.

"What's this?"

"Don't know...I think the beaker must not have been clean."

"Well...better get this cleaned up..."

"Yes sir..."

Of course he knew!