I've always used the spaghetti sauce metaphor. To the non-brewer I explain that, like homebrewing, there are many different levels of complexity when making spaghetti sauce.
The first, and simplest, is to take store bought spaghetti sauce, like Prego or Ragu, boil the noodles and combine. This is like the most basic form of extract brewing. The next, adding a little "flair" and complexity, is taking that store bought sauce and adding fresh tomatoes and herbs, making it a bit more of your own. This is like extract + specialty grains.
Next is taking ingredients like canned diced tomatoes, fresh tomatoes, your own herbs and spices and some tomato juice and combining them to create your own sauce. This is, technically, not completely home made, but it's damn close, just like partial mash.
Last is taking the tomatoes, fresh herbs and spices, garlic, etc. and cooking it all down yourself to make completely homemade sauce. This is like all grain.
I've used this metaphor for 10+ years to explain homebrewing to people and it seems to work well. I feel like a lot of homebrewers/people in general get it in their heads that homebrewing, like cooking, can't have certain levels of complexity.