All too often I see someone nursing a buoyant tea bag in a cup of hot water. The helpless bag bobs about only partially submerged, looking untidy. Sometimes the situation is improved with the help of a tea spoon to squash out the air but often it’s left unchecked.
A tea bag floats in water because the air surrounding the tea leaves was never given the chance to escape the bag before the paper became wet. Air won’t easily pass through wet paper. Once the bag is wet the air is trapped inside and will float indefinitely. The problem is compounded by the hot water raising the temperature of the trapped air, which causes it to expand like a balloon.
Rather than putting a tea bag at the bottom of a cup and pouring hot water all over it try putting the water into an empty cup then slowly lower the bag in. This gives the water a chance to soak through the bottom of the bag, displacing the air upwards and out through the dry and still breathable paper above the waterline.
If you lower the bag too slowly capillary action will cause the paper above the waterline to get wet before the air has escaped. If you lower the bag too quickly the air pocket will drop below the waterline before having a chance to escape through the dry bag, which is now submerged and wet. With practice you’ll find the balance point and your tea bags will sink every time. The bonus with this method is that you always have a firm grip on the string so it won’t end up in the drink.
Of course most civilised people make their tea in a tea pot or drink coffee instead.