After using the toilet, there’s something you should do before flushing: Shut the toilet lid! That’s because the force of water rushing from the bowl blasts a germ-filled mist containing pee and poo particles into the air.

Scientists have known about this phenomenon, called a toilet plume, for decades. However, it wasn’t well understood until recently. That’s because toilet plumes are invisible . . . or they were, until a team of researchers came up with a bright idea for studying them. They used a laser to illuminate the air above an open toilet bowl. When they flushed, the concentrated beam of light reflected off the particles that shot out of the toilet, revealing them in gross detail. “There was stunned silence in the room,” says John Crimaldi, a civil and environmental engineer at the University of Colorado Boulder who was part of the team. “We had no idea that we would see this eruption shooting almost 2 meters (6 feet) in the air.”

Toilet water contains disease-causing microbes like bacteria and viruses, says Crimaldi. “Now we know where those pathogens go when you flush.” Larger droplets settle on surfaces you might touch, like counters and doorknobs. Smaller ones float around in the air for minutes or hours, where you might breathe them in. And while closing the lid blocks some germs from escaping, the researchers say doing so doesn’t stop every particle.

Scientists aren’t sure how likely it is that pathogens in a toilet plume will make you sick. But now there’s no doubt that when you flush, droplets containing human waste will spray all over the place if you don’t close the lid.