How can you tell when your sister is on her period?
Your dad's knob tastes funny.
How can you tell when your sister is on her period?
Your dad's knob tastes funny.
Listen up, you dummy. So, the joke is saying that the way you know your sister is menstruating is because your dad is gettin' it on and his junk tastes like period blood. You probably think that is ok, you sicko. Sad!