Ocean waves affect an oil spill in two ways. They help carry the oil from its source to land—in this case, from the Deepwater Horizon drilling site in the Gulf of Mexico to Orange Beach on the Alabama shore—and they also churn the oil slicks into smaller globules that wash up on beaches and stick to sunbathers’ feet.

These so-called tar balls have a specific life cycle: Oil leaves the seafloor as a buoyant mixture of light and heavy hydrocarbons. In water, tar balls develop when sunlight evaporates out the more-volatile lighter hydrocarbons and the oil blobs develop a crusty shell as the oil oxidizes. The crust, which makes the tar ball “not unlike a toasted marshmallow,” as a National Oceanic and Atmospheric Administration fact sheet puts it, keeps the center of the tar ball soft. Tar balls can travel hundreds of miles, so beaches even farther from the spill may get oily visitors as well.