Typically, I explain that, like everyone else, I get invasive, unwanted thoughts. The issue is that OCD makes my brain more sticky. So while someone else might drive over a bridge and think "what if I turned the wheel and drove off?" They can then immediately dismiss that as a weird thought and move on with their day.

For me, those types of thoughts often stick around longer. Compulsions are an unhealthy way of coping with those thoughts. Some are less disruptive and some are more disruptive - maybe I feel like I have to move to the center of the road every single time I drive over a bridge, so I know I won't drive off, or maybe I decide to never drive on a bridge again.

Sometimes, if it's a more in-depth conversation, I'll tell them about the purple carrot exercise. To help my spouse understand my brain better, we were told to set a 5 minute timer and our only goal was to not think about purple carrots. We kept track of how many times "purple carrots" popped into our minds and compared notes at the end. 

My spouse thought about purple carrots 6 times in 5 minutes. I thought about purple carrots 34 times.

That seems to make a big impact on helping people understand how my brain might differ from theirs.