diff --git a/lib/coordinate.rb b/lib/coordinate.rb
index 49cdb29..01fd546 100644
--- a/lib/coordinate.rb
+++ b/lib/coordinate.rb
@@ -7,7 +7,8 @@ def initialize(x, y)
   end
 
   def distance_from(other)
-    # TODO: actually caluculate distance. I think we need to use the Pythagorean theorem?
-    0
+    dx = other.x - x
+    dy = other.y - y
+    Math.sqrt(dx*dx+dy*dy)
   end
 end
diff --git a/spec/shouty_spec.rb b/spec/shouty_spec.rb
new file mode 100644
index 0000000..38ea4b4
--- /dev/null
+++ b/spec/shouty_spec.rb
@@ -0,0 +1,21 @@
+require 'rantly/rspec_extensions'
+require 'shouty'
+require 'coordinate'
+
+describe Shouty do
+  it "only tranmits shouts within 1000m" do
+    property_of {
+      shouter_loc  = Coordinate.new(*Rantly(2) { range(0, 2000) })
+      listener_loc = Coordinate.new(*Rantly(2) { range(0, 2000) })
+      guard shouter_loc.distance_from(listener_loc) < 1000
+
+      [shouter_loc, listener_loc]
+    }.check { |shouter_loc, listener_loc|
+      shouty = Shouty.new
+      shouty.set_location('shouter', shouter_loc)
+      shouty.set_location('listener', listener_loc)
+      shouty.shout('shouter', listener_loc)
+      expect(shouty.messages_heard_by('listener').length).to eq(1)
+    }
+  end
+end