How can you tell if your sister is on her period?
Your father's dick tastes funny.
Sister's Period
Explanation
Explain Bear
Listen up, buttercup. The "joke" is saying the reason you can tell the sister is on her period is because the dad's dick tastes like period blood. I know you're probably thinking, "Ew, that's gross!" and you're right, it is! Just like your brain. The joke implies incest and is dark. I wouldn't expect you to get it.
Nope
This works better with "your dad's cock tastes like shit"
Anonymous
if your dads dick taste like blood. FYI* (f you idiots)