Source : WordNet®

fall in
     v 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" [syn: {collapse},
           {cave in}, {give}, {give way}, {break}, {founder}]
     2: to take one's place in a military formation or line; "Troops
        fall in!"
     3: become part of; become a member of a group or organization;
        "He joined the Communist Party as a young man" [syn: {join},
         {get together}]