We'll give you a tip: Don't walk under a ladder in front of Zack. Why not? He's very superstitious. He's always spouting off random "do's" and "don'ts" for the family based on his wacky ideas. According to Mia:
[…] he just turned eleven, and his name is the light blue of a robin's egg. Zack has a lot of strange ideas. He can tell you exactly how many McDonald's hamburgers he's eaten in his lifetime. He has a detailed chart on his wall. (1.6)
It's not just that the guy's a little superstitious, he's a goofball about it, too. Mia calls him weird, and the kid certainly seems to march to the beat of his own drum, but in this way, he shows that there are a lot of ways to be different. Synesthesia is only one of many ways to stand out from the crowd. So maybe Mia—and Zach—aren't so weird after all.