The answer to the crossword puzzle is ARTS. The phrase 'the 'A' that makes STEM into STEAM' refers to the addition of the ARTS to the acronym STEM, which stands for Science, Technology, Engineering, and Mathematics. By adding the ARTS, the acronym becomes STEAM, which stands for Science, Technology, Engineering, ARTS, and Mathematics. Therefore, the answer to the crossword puzzle is ARTS, as it is the letter A that makes STEM into STEAM.