Yes, the smell. And if it had been a pleasant smell, rather than cabbage or feet or worse, would you have performed better?
Quite possibly, according to an experiment carried out on nearly 100 students of physics at a high school in rural Missouri in the United States.
The researchers, Jeremy Watt and Cynthia MacGregor, of Southwest Missouri State University, decided to test the effect of a pleasant smell on the students' memory.
For four days in 2001, they varied the smell in the students' physics classroom. On days one and three, it was scented by burning a pine candle.
On days two and four, it was unscented. Students were asked five simple questions at the end of each physics lesson to see how much they had absorbed. On the scented days, they could recall far more.
Curiously, during the four-day study, only two of the 96 young scientists thought to ask why the candle was burning. And they appeared to accept the explanation that it was to remove the strong odours normally found in science classrooms.
By the fourth day, however, they seem to have smelled a rat (a change known to researchers as the Hawthorne effect). In other words, they had realised that all this testing was part of some kind of experiment and that they had better make more effort.
That is the most likely explanation, say the researchers, for the unexpectedly good results the students achieved in their tests on the fourth (unscented) day. The study, presented to the recent conference of the American Educational Research Association in Chicago, does not consider whether it might be the brain-clearing effect of the pine fragrance that had made the difference.
But it does concede that different smells could have different effects, and proposes a new study comparing the effects on memory of grapefruit, ginger and peppermint.
The research possibilities are endless.