Listen to part of a lecture in a biology class.
So, OK, we've been talking about frogs...and like all amphibians, frogs have thin skin, which means that they lose moisture through their skin easily. Now typically we think of frogs as living in wet environments, but for frogs who live in dry places with desert-like conditions, this can be a problem. Frogs have been able to survive in such areas by having different physical features, special dry-climate features that help them maintain an adequate level of moisture in their cells and avoid drying out. Some frogs do this by preventing water loss through their skin. By creating a sort of covering over their skin, they greatly reduce their skin's exposure to the dry air. The covering acts like a barrier that locks in moisture. For example, some frogs secrete a substance through their skin...a fatty substance that they rub all over their skin using their hands and feet, which creates a waxy layer all around their bodies that's almost completely watertight. Other frogs maintain an adequate level of moisture through a different physical feature...one that allows them to store water inside their bodies for later use. A specially modified internal organ inside their bodies enables them to have a high water-storage capacity. So the frogs are able to absorb and store moisture during wet, rainy times, which they can rely on to get through dry periods. The aptly named water-holding frog, for example, has a bladder that is highly elastic and stretchable. When it does rain, the frog absorbs water through its skin, and its bladder stretches to hold this extra water. The water is then slowly released from the special bladder into the frog's internal tissues until the next rain, which might not be for several months.
