Do You Know?

1. When a volcano explodes, it spews out lava. Where does lava come from ?
2. We often yawn when we feel sleppy. How does this come about ?
3. We hear of "Dry cleaning" clothes. How is it possible to clean clothes in a "dry" manner ?
4. I met someone who had "progressive" lenses in her spectacles. In what way were her lenses progressive ?