Simplify the following text for a Grade 9 native English-speaking person.
Original: Of the standard Î - amino acids , all but glycine can exist in either of two optical isomers , called L or D amino acids , which are mirror images of each other ( see also Chirality ) .
Simplified: There are 20 standard amino acids that almost all proteins are made out of . Some amino acids are glycine , alanine , and tryptophan .