If you fill a bucket with very wet mud, and then leave the bucket sitting on the ground, over time the more dense soil particles will sink to the bottom of the bucket due to gravity, leaving a layer of water on top. When you pick up a bottle of Italian salad dressing from the grocery store shelf, you see several different layers in the bottle. The dense solids have settled to the bottom, the vinegar forms a middle layer, and the least dense oil is on top. Gravity can also mask some phenomena that scientists wish to study. An example is the process of diffusion.
Diffusion is the intermingling of solids, liquids, and gases due to differences in composition.
Such intermingling occurs in many situations, but diffusion effects can be easily hidden by stronger convective mixing. As an example, imagine a large room in which all air circulation systems are turned off and in which a group of women are spaced ten feet apart standing in a line. If an open container of ammonia were placed in front of the first woman in line and each woman raised her hand when she smelled the ammonia, it would take a considerable amount of time before everyone raised her hand. Also, the hand raising would occur sequentially along the line from closest to the ammonia to furthest from the ammonia. If the same experiment were performed with a fan circulating air in the room, the hands would be raised more quickly, and not necessarily in the same order.