What's black and sits at the top of a staircase? Stephen Hawking after a house fire
Is this a joke about handicapped people?
Yeah, they're useless and would be burned to a crisp in a fire. They're completely incapable.
It's cruel to make jokes about circumstances people can't control. I imagine you wouldn't want someone laughing at your inability to perform simple tasks. Please refrain from treating handicapped people like laughing stocks.