Tea-bagging

The wonderfully urbane and erudite Dr Karl continues to facilitate the uncovering of some delightful scientific miscellany on his Radio 5 phone-in. A listener called last week to ask why it is that, when you pour boiling water onto a tea-bag already placed in the cup, the tea-bag will inflate, and rise buoyantly with the water, to the top of the cup, whilst if you fill the cup with boiling water first, and then place the tea-bag into the cup, the tea-bag will generally sink beneath the surface of the water. (Try it!)

Whilst Dr Karl was bereft of an answer on this occasion, another listener reported this week that if you re-boil a kettle of water, and repeat the experiment with the tea-bag already in the cup, then the tea-bag will not inflate under these conditions. The explanation inferred from this is that the tea-bag is inflated on the first occasion by oxygen dissolved in the water; when the experiment is repeated, there is no longer a sufficient amount of dissolved oxygen in the water.

So, I exhort you all to re-fill your kettles to avoid the disappointment of a non-inflating tea-bag. Also, as ever, please refrain from drinking coffee at all times.