Why are certain amino acids called essential amino acids?

Adigestive system that is a series of tubes beginning at the mouth and ending at the anus is a digestive system.

Ricardo notices that when he plays soccer in the rain, his ball does not seem to go as far after a kick. what is the best scientific question that ricardo could ask about this observation? is soccer better played in the rain or sun? how does rain affect the distance a soccer ball can travel? how does rain affect the speed of a soccer ball? is soccer affected by the rain?

In general, characteristics that an organism survive and reproduce become more common over time. what mechanism of evolution causes this?