"Head back to the Training menu, this time picking the Spy class.
"The Spy can turn invisible for a small period of time by right clicking. After a friendly spy demonstrates, follow suit and sneak by the Red sentry while invisible.
"The Spy can also disguise himself as any class from the enemy team to fool players and Sentries. Disguise yourself and approach the sentry.
"Now you learn how to safely destroy Sentries (and other buildings made by the Engineer). Pull out the Sapper weapon and attach it to the Sentry to disable it and slowly destroy it. Notice how placing a Sapper doesn't make you lose your disguise.
"Finally, you will learn the Spy's most powerful move, the Backstab. Pull out your Knife and walk up behind the bots. If your arm raises, it means you are lined up for a Backstab that will INSTANTLY KILL any player unless they are Ubercharged. Note that atattacking will remove your disguise. Kill those three bots (including the enemy Spy disguised as a friendly teammate) and move onto the Final Training!