We often take clean drinking water for granted, but we should think about where it comes from. The Potomac River has been supplying the drinking water for much of the Washington area for 150 years. Everyone can help keep drinking water safe by making sure the Potomac and other nearby rivers are as clean as possible.

KidsPost and the Nature Conservancy, which helps protect rivers and streams, are teaming up to find out what readers have done or plan to do to keep drinking water clean and how it helps. (For ideas, go to www.nature.org/kidsandwater.)

Fill out an entry form at ­kidspost.com or write us a letter of no more than 200 words and send it to KidsPost, The Washington Post, 1150 15th St. NW, Washington, D.C., 20071. Entries are due September 10.

Four semifinalists and their families will be invited to a picnic and hike with a Nature Conservancy scientist September 21 at the Potomac Gorge in Maryland. At the hike, the contest winner will be announced. The winner will receive a behind-the-scenes tour for four at the National Aquarium in Baltimore. We will publish the semifinalists’ entries in KidsPost.

The contest is open to kids ages 7 to 13 who live in the Baltimore-Washington area. One entry per child. You need to have an adult (a parent or teacher) send us your entry. The adult should include your name, age and home town as well as the name and phone number of the adult submitting the entry. A note from the adult giving permission for you to enter the contest is also required.