I don't know how that got there
You say you don't know how it got there because you're the one who put it there and you're too scared to get yelled at.
"I don't know how that got there", after throwing a shoe at your brother.
"I don't know how that got there", after knocking over the trash can.
"I don't know how that got there", while eating pizza from the floor and wearing a sock on your head.