The only thing I liked about Captain Kangaroo is that one of the
puppets (I believe it was the moose) would consistently trick the
Captain into saying a phrase which caused seemingly hundreds of ping
pong balls to drop on the Captain's head.  I believe it was the same phrase
every time.  WHAT WAS THE PHRASE?

Quoting Bob Keeshan himself: 
(from the book "Good Morning Captain," from Fairview Press)

"I am often asked what word or signal Mister Moose engaged 
to trigger the fall of those spheres. There were many cues, of 
course, ranging from knock-knock jokes to rhymes to riddles."

Two examples from the book:

"Roses are red, 
Violets are blue,
I like it most,
When ping-pong balls fall on you!"

"Knock, knock."
     "Who's there, Mister Moose?"
"Alaska."
     "Alaska, who, Mister Moose?"
"Alaska to duck, 'cause here they come!"

Source: Google Groups
http://groups.google.com/groups?selm=336f3972.4790692%40nntp.ix.netcom.com&output=gplain


Captain Kangaroo Trivia
 
?What word made the ping pong balls drop?
Well, this is not a memory loss issue! There was no one word. The
punch line or point of the gag made them drop. Usually a knock-knock
joke. ?
http://www.fiftiesweb.com/tv/captain-kangaroo.htm

?Mr. Moose asks the Captain a knock-knock joke (the dropping of balls
comes along with the punchline - usually "ping pong balls".
http://www.tvparty.com/lostterrytoons.html

The punchline of Mr. Moose?s  riddles and knock-knock jokes ended with
dozens of ping-pong balls hitting the Captain in the head.?
http://www.wordiq.com/definition/Captain_Kangaroo


?Mr. Moose regularly confronted the Captain (Bob Keeshan) with
knock-knock jokes and dropped Ping-Pong balls onto the head of his
soft-spoken benefactor.?
TV Acres
http://www.tvacres.com/deers_mrmoose.htm

?Actually, the ping-pong balls always came down in a type  of either a
knock-knock joke or some kind of a joke with  a phrase of something
coming or falling.

Example........
Mr. Moose :  Knock, knock !!
Captain:     Who's there?
Mr. Moose:   Reddy !
Captain:     Reddy who??
Mr. Moose:   Reddy or not here they come!!!
 (At this point ping-pong balls fall down from ceiling   onto Captain Kangaroo)?

Source: Google Groupshttp://groups.google.com/groups?selm=bigalDxnGp8.p2%40netcom.com&output=gplain


Here are a few more examples of the ping-pong ball knock knock jokes
on Captain Kangaroo.

Knock, knock. 
Who's there? 
Jessie. 
Jessie who? 
Do you see any ping pong balls around here? 
CNN.com - Transcripts 
http://www.cnn.com/TRANSCRIPTS/0401/24/smn.01.html


Knock-knock. 
Who's there? 
Candy? 
Candy who? 
Candy ping-pong balls be held back this time? 
http://nothingbutcool.typepad.com/nbc/2004/01/gbye_captain.html


?We learned to laugh at the antics of Mr. Moose who entertained us with
his jokes and would trick the Captain usually by telling a "Knock, Knock"
joke which went something like this:
     "Knock, knock."
     "Who's there?"
     "Justin."
     "Justin who?"
     "You're justin time!"
At those words, hundreds of ping-pong balls would fall on the
Captain's head.?
http://archive.mail-list.com/heartwarmers/msg01192.html

"Knock-knock!"
    "Who's there?"
    "Mini."
    "Mini-mini ping-pong balls!"
    ***CRASH!***

Source: Google Groups
http://groups.google.com/groups?q=%22knock+KNock%22+%22who%27s+there%22+ping+pong+balls+kangaroo&hl=en&lr=&ie=UTF-8&c2coff=1&selm=401266A5.5040807%40wizvax.net&rnum=1

Hey, Captain."
"Yes, Mister Moose?"
"Wanna hear a knock-knock joke?"
"Well, I don't see why not ... "
"Knock-knock."
"Who's there?"
"Lotta."
"Lotta who?"
"Lotta ping-pong balls!"
<shower of ping-pong balls

Source: Google Groups
http://groups.google.com/groups?selm=5FF9913361A1F89E.C6737DC97149D98D.DDDE242E594533E8%40lp.airnews.net&output=gplain


Search criteria:
Mr. Moose Captain Kangaroo ping pong balls
"knock knock" "who's there" ping pong balls kangaroo

I hope this answers your question.


Best regards,
Bobbie7