How do you explain MS to those who don't have it? How do you explain why you have so many limitations when they aren't always apparent and obvious?

The reason I'm asking is because of a comment someone made to me in an email. I haven't seen this person (a family member) in a long while so they aren't familiar with my daily struggles and limitations.

After I explained that I no longer work and am living on SSDI they wanted to know how they could "get on that gravy train".

I haven't responded and won't until I'm a little less offended. How can folks be so clueless? I really don't owe them or anyone else for that matter an explanation. But I always seem to have trouble describing my condition to those who aren't familiar with it. It would take all day to go over all the different sx I deal with.