A lecture about English
A linguistics professor was lecturing to his English class one day. "In English," he said, "A double negative forms a positive. In some languages, though, such as Russian, a double negative is still a negative. However, there is no language wherein a double positive can form a negative."

A voice from the back of the room piped up, "Yeah, right."

A U.S Army Colonel issues a directive
A U.S. ARMY COLONEL ISSUED THE FOLLOWING
DIRECTIVE TO HIS EXECUTIVE OFFICERS:

"Tomorrow evening at approximately 2000 hours Halley's Comet
will be visible in this area, an event which occurs only once every
75 years. Have the men fall out in the battalion area in fatigues,
and I will explain this rare phenomenon to them. In case of rain,
we will not be able to see anything, so assemble the men in the
theater and I will show them films of it."

EXECUTIVE OFFICER TO COMPANY COMMANDER:

"By order of the Colonel, tomorrow at 2000 hours, Halley's comet
will appear above the battalion area. If it rains, fall the men out in
fatigues, then march to the theater where this rare phenomenon
will take place, something which occurs only once every 75 years."

COMPANY COMMANDER TO LIEUTENANT:

"By order of the Colonel in fatigues at 2000 hours tomorrow evening,
the phenomenal Halley's Comet will appear in the theater. In case
of rain, in the battalion area, the Colonel will give another order,
something which occurs once every 75 years."

LIEUTENANT TO SERGEANT:

"Tomorrow at 2000 hours, the Colonel will appear in the theater with
Halley's Comet, something which happens every 75 years if it rains,
the Colonel will order the comet into the battalion area."

SERGEANT TO SQUAD:

"When it rains tomorrow at 2000 hours, the phenomenal 75 year old
General Halley, accompanied by the Colonel, will drive his comet
through the battalion area theater in fatigues."