diff --git a/tests.lua b/tests.lua
index f23b4b6..e42b037 100644
--- a/tests.lua
+++ b/tests.lua
@@ -137,7 +137,7 @@ function buildEnvironment(testString)
    return testEnv
 end
 
-local twoRandom = function() return math.random(100000), math.random(100000) end
+local twoRandom = function() return math.random(-100000, 100000), math.random(-100000, 100000) end
 
 
 describe("Interpreter tests", function()
@@ -186,7 +186,7 @@ describe("Interpreter tests", function()
       end)
       it("can evaluate a arithmetic expression with multiple operations", function()
          for i = 1, 1000 do
-            operands = {math.random(100000), math.random(100000), math.random(100000), math.random(100000)}
+            operands = {math.random(-100000,100000), math.random(-100000,100000), math.random(-100000,100000), math.random(-100000,100000)}
             local operations = {math.random(4), math.random(4), math.random(4)}
             local testString = ""..operands[1]
             local expectedResult = operands[1]
@@ -211,6 +211,27 @@ describe("Interpreter tests", function()
          end
       end)
    end)
+   describe("Core word tests", function()
+      describe("DUP tests", function()
+         it("DUPs the top item of a stack with one item on it.", function()
+            local testEnv = buildEnvironment("1 DUP")
+            interpreters.start(testEnv)
+            assert.are.same(1, testEnv.activeDataStack.contents[2])
+         end)
+         it("DUPs the top item of a stack with random quantities of random numbers on it.", function()
+            for i = 1, 1000 do
+               local stack_depth = math.random(100)
+               local stack_top = 0
+               for j = 1, stack_depth do
+                  stack_top = math.random(-100000,100000)
+                  testEnv.activeDataStack:push(stack_top)
+               end
+               local testEnv = buildEnvironment(stack_top.." DUP")
+               assert.are.same(stack_top, testEnv.activeDataStack.contents[j+1])
+            end
+         end)
+      end)
+   end)
 end)