Using redirect_to with a specific ActiveRecord object to create a link to that object

I'm going through Michael Hartl's tutorial at http://ruby.railstutorial.org/. It's basically a message board app where Users can post Messages and others can leave Replies. Right now I'm creating Users. Inside the UsersController things look like this:

    class UsersController < ApplicationController
      def new
        @user = User.new
      end

      def show
        @user = User.find(params[:id])
      end

      def create
        @user = User.new(params[:user])
        if @user.save
          flash[:success] = "Welcome to the Sample App!"
          redirect_to @user
        else
          render 'new'
        end    
      end
    end

The author says that the following lines are equivalent. Which makes sense to me:

    @user = User.new(params[:user])
    is equivalent to
    @user = User.new(name: "Foo Bar", email: "foo@invalid",
             password: "foo", password_confirmation: "bar")

redirect_to @user redirects to show.html.erb. How exactly does that work? How does it know to go to show.html.erb?