Loons have big air sacs inside their body. When they breathe in, they can fill these air sacs to capacity. This expands their bodies to be a little bigger, and makes loons float. When they breathe out, they usually keep the air sacs partly full. But they can push out most of the air in their air sacs, making their bodies a little smaller and helping them to sink. If a predator approaches, a loon can quietly sink into the water without leaving a ripple. If it's chasing a fish fairly deep in the water, the partly deflated air sacs make the loon dense enough to easily stay underwater for over a minute before it needs to come up for air.To understand how loons can use their internal air sacs to sink and float, try this experiment:

Materials Needed

• regular balloons
• duct tape
• some rocks
• a deep sink.

1. Working in pairs or small groups, different students can use different rocks. They should weigh the rocks first, and guess how many balloons it will take to make each rock float.
2. Next, blow up the balloons, tie them off, and tape one to your rock. Does it sink? How far down?
3. Keep sticking on new balloons with duct tape and see what happens.
4. Can you find a rock that weighs 6 to 8 pounds, which is the weight of a loon? Guess how many balloons are needed to support that rock, and then experiment as you record your answers in your journal:
   • Was your guess too high or low, or just right?
   • Using a ruler, try to estimate the balloon's and the rock's volume. How much volume do you think a loon's air sacs would have compared to the loon's solid body parts?
   • What other adaptations besides air sacs help loons to float?
   • Now think about this: Which weighs more: a pound of feathers or a pound of rocks? If they both really weigh the same, which would you rather have fall on your head? Why?