Done

first.ru

@@ -0,0 +1,9 @@
+require 'rack'
+
+# Instances of Proc automatically have a call method that runs the block that
+# they're initialized with.
+my_server = Proc.new do
+    [200, { 'Content-Type' => 'text/html' }, ['<em>Hello World!</em>']]
+end
+
+run my_server

my_server.rb

@@ -0,0 +1,9 @@
+class MyServer
+    def call(env)
+        return [ 200, {'Content-Type' => 'text/html'}, pretty_response ]
+    end
+
+    def pretty_response
+        (Time.now.to_i % 2).zero? ?  ["<em>Hello</em>"] : ["<strong>Hello</strong>"]
+    end
+end

second.ru

@@ -0,0 +1,11 @@
+class MyServer
+    def call(env)
+        return [ 200, {'Content-Type' => 'text/html'}, pretty_response ]
+    end
+
+    def pretty_response
+        (Time.now.to_i % 2).zero? ?  ["<em>#{Time.now} - Hello</em>"] : ["<strong>#{Time.now.to_i} - Hello</strong>"]
+    end
+end
+
+run MyServer.new

third.ru

@@ -0,0 +1,2 @@
+require_relative './my_server'
+run MyServer.new