Miley wrote: "Just realised my soap wasn't working because it's literally a block of cheese.
"I use a liquid soap for the dishes, bar soap like dis is gentler on my hands after I do chores if that makes any sense [sic]."
But Miley admitted alcohol may have played a factor in her blunder.
She said: "It was a couple days of 'Why isn't this foaming?!'
"I come to realise it was a dried-out square of Tillamook sharp cheddar cheese.
"I suspect I left it out when I was intoxicated and just forgot."
Her post has gone viral, with others poking fun at Miley's mishap.
One asked: "But that must mean ... have we been eating Mac&Soap the entire week??"
"Does that mean you put the soap in the fridge?" another questioned.
"To be fair to you it does look like a block of soap."