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Place the balloon-covered end over your heart (or someone else's heart), and the other end against your ear. If you used a second funnel, make sure that the funnel is touching your head; if you left the tube as-is, gently stick the end into your ear, like an earbud. If the room is quiet enough, you should be able to hear the heart beating!  If you can't hear the heart, do some jumping jacks for about 1 minute, then try again. If you're listening to someone else's heart, have them do the jumping jacks. A quiet room is important. Because this is a handmade stethoscope, it's not very strong or loud. A noisy room will drown out the heartbeat.
Use the stethoscope in a quiet room.