diff --git a/intercepting-communication/udp-2/DESCRIPTION.md b/intercepting-communication/udp-2/DESCRIPTION.md
index b5e0aa6..a23198a 100644
--- a/intercepting-communication/udp-2/DESCRIPTION.md
+++ b/intercepting-communication/udp-2/DESCRIPTION.md
@@ -1,2 +1,9 @@
 Though we didn't explore this for TCP, in addition to selecting the destination port, both TCP and UDP can set their _source_ port.
 We'll practice that here --- you can set the source port with `s.bind` on the socket, exactly how a server does it to set their listening port.
+Read the source code of `/challenge/run` to see what source port you'll need!
+
+----
+
+**NOTE:**
+You must set the source port _before_ sending data!
+Otherwise, Linux will pick a random source port (the default behavior, when `bind` is not called).