fall down

verb

1. To fall to the ground. To collapse.

   “Ring a-ring o' roses, / A pocketful of posies. / A-tishoo! A-tishoo! / We all fall down. — traditional nursery rhyme (British version)”

   “The beams supporting the roof had rotted, causing the entire house to fall down.”

2. To sail or drift toward the mouth of a river or other outlet.
3. To fail; to make a mistake.

   “That is where your reasoning falls down.”