What is the difference between student-centered learning and teacher-centered learning?

A teacher-centered learning environment holds the teachers as the focal point of the classroom. The teacher usually is giving lectures and demonstrations TO students. The students are passive participants. A student-centered learning environment the students are the center of the classroom. Students are actively engaged in their own learning and are usually researching, solving problems, critically thinking, communicating, and collaborating with each other.

Why is obsidian considered to be a extrusive, igneous rock.

Igneous rocks are classified as two types: extrusive or intrusive. Intrusive rocks form and crystallize from magma beneath the Earth's surface. Imagine a thermos of hot water and an open cup of hot water. Which cools more slowly? We would expect the thermos with hot water to cool more slowly as the Earth acts as a thermos. The magma cools very slowly allowing large crystals to form and produces rocks with a coarse-grained texture. Coarse-grained means rough because the crystals are large. Now extrusive igneous rocks cool from lava on the surface of the Earth. Since cooling is very rapid crystals do not have as long of a time to form and are very small. This creates fine-grained igneous rocks. With obsidian, lava cools so rapidly because it is usually quenched in water. This is what provides the glassy texture of obsidian and why we call it an extrusive, igneous rock.

How do mutations arise?

A change in the DNA or RNA sequence is called a mutation. Mutations in our genetic code happen all the time. Some changes occur spontaneously without any outside influence, like Downs syndrome. Yet others are caused by external factors. External factors like UV exposure from the sun can create skin cancer in some individuals. Mutations also differ in the way the genetic material is changed. They can change the structure of the entire chromosome or just change a single nucleotide varying the overall effect of the mutation. In the next section, we will discuss the various ways mutations can occur and I will provide specific disorders that arise from different types of mutations.