Long before I went into architecture, I did a lot of backpacking and climbing, using USGS maps, so I was genuinely surprised to discover later that not everyone had learned this as children.

Boy Scouts for me, but same idea.

We had a second or third year landscape architecture course in my curriculum that was required for architecture students. One of the activities to help reinforce the understanding of contour lines was to draw one foot interval contour lines around campus on the sidewalks, drives, etc. with sidewalk chalk. The group I was in was told to draw the contours near a planter. So we started by laying out the horizontal lines one foot apart on the vertical wall of the planter, which made it really simple to see where they would fall as the planter hit the sloping sidewalk. As we were finishing up, some guy who thought he was all smart started making fun of us for drawing the lines on the vertical wall because "that's not how contour lines work" or something like that. The professor overheard and said, "no, that's exactly how contour lines work." The image above reinforces that.