Some things are nearly invisible when working well. When not working well, their critical function is glaringly obvious.

I called our bank to find out why our debit cards stopped working. The phone connections were so bad on my first two attempts that I ended the calls. I had marginal success on my third call. We could understand each other just well enough to order me a new card. It made me realize how much I take clear phone signals for granted.

Our vestibular systems (the key to our sense of balance) are like this. When working well, you don’t even think about moving about without concern. When things go awry, you may become debilitated with almost no warning or need to avoid things like riding in the back seat of a car to avoid nausea.

Lower back muscles are another linchpin. Normally you don’t think about walking around upright and picking items up off the floor. Strain these muscles and you suddenly realize you use them for everything. A couple of years ago, a back muscle strain had me struggling to walk or even get out of bed. At one point, it took me 15 painful minutes to get from the bed to the bathroom.

What did you take for granted that you realized is critical when it failed to work as intended?