What is the thing called that you put loose leaf tea in?

Often confused with a strainer, a tea infuser is a ball or basket that you put your tea leaves in while they brew. The point is easy removal of the leaves once you're done brewing your tea. You don't want your brewed tea leaves in the water too long because they will continue to brew and may turn your tea bitter.

A tea infuser is a device in which loose, dried tea leaves are placed for steeping or brewing, in a mug or a teapot full of hot water; it is often called a teaball or tea maker, and sometimes a tea egg. The tea infuser gained popularity in the first half of the 19th century. Tea infusers enable one to easily steep tea from fannings and broken leaf teas.

A tea infuser performs a similar function as a tea bag, a later American invention. The infuser is generally a small mesh or perforated metal container or covered spoon that holds tea leaves, in varying sizes to steep single or multiple servings at once. Common shapes for infusers include spherical, conical and cylindrical. One style of infuser is a split sphere with tong-like handles to open its mesh container.
The infuser is placed in a cup or pot of hot or boiling water, allowing the tea to brew without loose tea leaves spilling into the pot or cup. A rod or chain is commonly attached to the container of the infuser to simplify retrieval from the pot or cup. Infusers with large holes may not catch all the leaves, requiring the use of a tea strainer to remove the remaining pieces.
An infuser that produces more gravity than the buoyancy of floating material it carries will sink. And pressure increases with depth as a result of the weight of the overlying fluid. Thus, the pressure at the bottom of a tall container of fluid is greater than at the top of the container.