Sentence:  In a chastened mood, Helen returned to her own room and lit her spirit-lamp, in order to re-boil the coffee. She was watching the brown bubbles <WSD>foam up</WSD> in the saucepan, when she heard the front-door bell. 
Meaning of the word marked with <WSD> tags:  To become foamy, create a foam; to rise with a foamy surface or covered with something resembling foam