fall in in WordNet English dictionary

1. break down, literally or metaphorically
   "The wall collapsed"; "The business collapsed"; "The dam broke"; "The roof collapsed"; "The wall gave in"; "The roof finally gave under the weight of the ice"
   collapse  fall in  cave in  give  give way  break  founder 
   
2. become part of; become a member of a group or organization
   "He joined the Communist Party as a young man"
   join  fall in  get together 
   
3. to take one's place in a military formation or line
   "Troops fall in!"
   fall in 
   

fall in in English Wiktionary