When you turn 18 your soul marks appear, however many you have is different to each person. Each mark is different and it will tingle when you are close to the person it belongs so. When you touch them, you both become connected. You may chose to hide them, so even if you touch, you won't connect and they will not tingle when you're around them. However this is highly dangerous and discouraged as you don't get serotonin or dopamine. Zia has 13, and wants nothing to do with them.