A hazy, greasy-looking layer floating on your tea’s surface and that same layer breaking apart into sea ice-like "chunks" when you stir the drink can be a frustrating problem.

Luckily, that film doesn’t make your tea unsafe to drink or affect its taste or quality. Per a study in the journal Physics of Fluid, it forms when tea is brewed using hard water.

Hard water is water with a very high concentration of soluble minerals. When the chemicals in the tea react with those minerals, that reaction causes the film to form.

The film is initially too thin to be seen with the naked eye, but it becomes visible after about 30 minutes of cooling. Thankfully, scientists have found ways to get rid of it.

It’s best to use filtered, soft water that has no minerals. If that’s not feasible, add some acidity, like lime or lemon juice, to your tea to lower its pH and keep the film thin.

Note that the other ingredients in your tea can also affect the film's formation. While a small spoon of sugar helps thin it out, milk thickens it — even when used in tiny amounts.