If the upper atmosphere is is very thin, why do space vehicles heat up as they enter the atmosphere?

What are genotypes? what are phenotypes?

Many organizations release indexes used to measure the development of the world's countries. as we learned in this lesson, these indexes measure many factors, from life expectancy to happiness. in your opinion, what are the three most important factors we can use to determine how developed a country might be? explain your answers in a few sentences.

What do transitional fossils best support?