diff --git a/nth-prime/example.rb b/nth-prime/example.rb
index 923ebda9b3..6a8f1b1152 100644
--- a/nth-prime/example.rb
+++ b/nth-prime/example.rb
@@ -1,5 +1,3 @@
-require 'prime'
-
 class Prime
   def self.nth(n)
     if n < 1
@@ -11,8 +9,17 @@ def self.nth(n)
     i = 1
     while primes < n
       i += 1
-      primes += 1 if Prime.prime?(i)
+      primes += 1 if self.prime?(i)
     end
     i
   end
+
+  def self.prime?(n)
+    return false if n == 1
+    return true if n == 2
+    (2..Math.sqrt(n).ceil).each do |i|
+      return false if n % i == 0
+    end
+    return true
+  end
 end
diff --git a/nth-prime/nth_prime_test.rb b/nth-prime/nth_prime_test.rb
index 9034de1078..8dfdbc75fe 100644
--- a/nth-prime/nth_prime_test.rb
+++ b/nth-prime/nth_prime_test.rb
@@ -1,4 +1,21 @@
 require 'minitest/autorun'
+
+require 'prime'
+Error_message = "Using Ruby's Prime class is probably the best way to do this in a 'real'\n" +
+  "application; but this is an exercise, not a real application, so you're\n" +
+  "expected to implement this yourself ;-)"
+class Prime
+  [:each, :new, :prime?, :take].each do |m|
+    define_method(m) { |*_| raise Error_message }
+  end
+end
+
+class Integer
+  [:prime?, :each_prime].each do |m|
+    define_method(m) { |*_| raise Error_message }
+  end
+end
+
 require_relative 'nth_prime'
 
 class TestPrimes < Minitest::Test