walk the chalk
To show one's competence in a particular area. This outdated phrase refers to a sobriety test in which one had to walk between chalk lines. I was so worried that I wouldn't be able to walk the chalk, but I got a perfect score on my exam!
walk the chalk
have your sobriety tested. A traditional method of ascertaining whether someone is sober or not is to see whether they can walk along a line chalked on the ground without wobbling.