The girl holding the test tubes is a student.  Her teacher is watching her perform a simply chemistry experiment to assess her lab skills.  The girl is called Susan, she's feeling very nervous and tense with her teacher watching her.  Her teacher is trying to keep as quiet as possible, allowing Susan to perform the experiment in peace.    Susan holds the test tube steadily in her hand and pours the second chemical in with the dropper.  The chemical in the test tube reacts with the new chemical and fizzes.  Susan writes down the results in her paper and slowly puts the test tube away.  The teacher is pleased with her performance and gives her an A for the assessment. 
