I Know That You Know

I Know That You Know is the 21st episode and Season 3 of The Cosby Show, also the 70th overall episode in the series. The episode was directed by Jay Sandrich and written by John Markus, Carmen Finestra, and Matt Williams. The episode originally aired in the United States on NBC on March 19, 1987.

Synopsis
When Sondra and Elvin announce their engagement, Clair and the children use the news to play a practical joke on Cliff, who launches a counter-offensive of his own.

Plot summary
Sondra and Elvin announce their engagement. Since Cliff always says he knows everything that goes on, Clair and the kids decide to play a joke on Cliff about the engagement. Anyway, Theo who received a package of practical jokes in the mail, succeeded in fooling Clair, Vanessa and Rudy. But Theo's gags could'nt fool Cliff, and he brags that he's too clever to be tricked. Later, when Cliff is out, Sondra and Elvin visit the family to announce their engagement. The Huxtables decide to use the exciting news to play an elaborate joke on Cliff. He finds out before the joke before the rest of the family can put it into action, and then turns the tables on the others.

Note: Lisa Bonet does not appear.