The teaching assistant steps in to help a trio of Year 10 boys with a test to see how sugar affects yeast respiration in dough.
"This is going to be championship yeast, I tell you," he announces, vigorously shaking the tube. The dough refuses to rise.
The assistant is David Miliband, school standards minister, who this week played at being a teaching assistant at Kemnal technology college, near Sidcup, Kent.
Despite his difficulties with the yeast, he was described as "very useful" by Grant Gill, the science teacher who supervised him helping pupils in groups and inputting experiment results onto a laptop.
"He was better than a few teaching assistants I have had," Mr Gill said.
"He got straight in there and had a good rapport with pupils."
Mr Miliband's enthusiasm impressed Andy Parry, the information technology teacher, even though the minister hijacked his lesson to show pupils the British Museum's website.
Later, Mr Miliband said he had carried out only a handful of tasks assistants should cover but felt the experience validated his belief that support staff were at the centre of a "quiet revolution" in schools.
"I was impressed the pupils recognised that teachers and teaching assistants could help them in different ways and did not see them as interchangeable.
"And I think those I worked with on the yeast learned that scientific experiments do not necessarily go as expected."