LONDON, ENGLAND - We don't know how the guards could keep a straight face, because we sure couldn't!
A Buckingham Palace guard was left red-faced when he tripped on a manhole cover during the Changing of the Guard.
The guard lost his footing and knocked his bearskin on the side of the box, then he dropped his rifle.
The guard jumped to his feet, and his other guard friends just kept marching as if nothing happened.
Watch the hysterical video here.