. >> reporter: showing students what happens after you flush is what marty johnson does. >> everybody pay attention. you should have a slide and a pipette. >> reporter: these budding scientists spent the day at washington suburban sanitary commission plant to learn what it takes. >> that's actually called a rotor. >> reporter: to keep the tap running clear. >> the dark stuff goes to the bottom and the clean stuff goes to the top. >> bacteria is actually the worker bees in the process. they're doing the job, the things that are in that sludge. >> reporter: the science behind the sludge is important not only for public health, but for inspiring future scientists in the classroom and in the real world. >> it also gives them enough information so when they go into biology next year, they'll have a better understanding of what does it mean as a bioindicator meaning a living thing that we can use to sort of tell us what the bacteria levels are. >> they get to a point they see the process, they like hands on, they like learning. >> reporter: for some of these gaithersburg high school student