Overthinking riddles

"What is lighter than a feather but we cannot hold for more than a few seconds, please answer in the chatbox" quipped our science teacher as she glowed with the satisfaction that one gets when they find a way to make that class fun. We waited politely for a minute and then filled the chatbox with 45 'breath'. Delighted at the intellect of her students she then showed us a walnut and asked us to write what it is.
We are currently studying the brain and as 10th class students we know that questions which seem simple are actually not, for example; the question may ask the speed of an apple falling from a tree however the answer should contain the rate of photosynthesis occurring in the neem tree beside the apple tree.
So naturally, most of us wrote 'brain', a few overachievers- 'left and right side of the brain', creative kids- 'lungs'.
"it's a nut," said our teacher with a glint in her eye.
After spending a second absorbing the fact that the question we heard wasn't a clever disguise worn by a much tougher question, we all hesitantly wrote walnut (except a few kids who threw in groundnut and a Pista as a precaution)
Having started on a joyous note we now proceeded with the class.

 Nothing like a visual teaser to begin a tough 1 hour-long lesson.