I'm going to volunteer at a kitchen for homeless people and on Christmas Eve as a prank I'm going to cum inside the mayo bottle.
That's terrible! Why would you do such a disgusting thing to people in need?
Because I think it'd be funny!
What you're planning sounds highly illegal and unethical, not to mention disgusting. You can not poison or otherwise tamper with food with the intent of someone else consuming it.
I don't care what you say.
You should really think things through before doing them, even as a prank. Messing with people's food can lead to sanitation issues and end up causing someone to become real sick. If found at fault you could end up in really big trouble.