I try, but it doesn't always work. This also happens with other jokes I have that I am completely dead pan about. My belief is that what I'm saying is so ridiculous that it should be obvious I'm not serious, but some people are… um… for instance, I have a fake conspiracy theory that plane flight doesn't exist, and airlines just knock us out, hook us up to electrodes, and wheel planes into a hangar where we get teleported. Planes seen in the sky are government holograms. I've had (stupid and annoying) people argue with me that I was in fact seriously stating my beliefs and then just trying to weasel out of it when I explained I was obviously joking.

Edit to add, one basis for that plane conspiracy is that lift is fake since scientists don’t know why it works. “That air moves faster over the bottom than the top so it makes this 80 ton tin can float in the air”? Yeah, right.