Passover is a holiday near and dear to Marc Jaffe's heart. So when the "Seinfeld" and "Mad About You" writer went to a friend's house for a seder last year, he was let down when an Elijah's entrance gag bombed.

"They shook the table. I thought, 'You gotta be kidding me,'" he said. "You gotta have better effects than that."

This year Jaffe is ready with a Pesach trick of his own. It's an illusion that might leave guests guessing (or groaning). He has created a cup that "drinks itself."

The cup sits on the Passover table filled with wine during the first part of the seder. When it's time to open the door for Elijah, the host picks up the aluminum cup, invites Elijah in and declares that this year Elijah will drink.

Normally after Elijah is invited in, the contents in the cup don't move until it is time to wash the dishes. But with Jaffe's creation, the wine slowly disappears into a secret compartment after the cup is set down.

In a video on his elijahdrinks.com Web site, Jaffe demonstrates his invention to a friend. Because he wanted a genuine reaction, he had a set-up that sitcom writers seldom encounter: "I had to do it in one take." (Thus Jaffe mentions in the video that it sells for $29.95, when it's actually priced at $34.95 plus shipping on the Web site.)

Jaffe developed the Elijah Drinks cup with friend Kerry Pollock, a comedian as well as a magician who has performed at the Magic Castle and in Las Vegas. Each cup is handmade, and Jaffe says a blessing over each one before he tests the trick lever that makes the gag possible.

Although his wife and three daughters are excited about the cup, Jaffe says they had mixed reactions while he was creating it.

"[They said] 'Why are you wasting your time doing that?'" said Jaffe, author of the 2000 humor/health book "Sleeping With Your Gynecologist" (that would be his wife). "My kids generally ignore me -- 'Just another crazy thing Dad is getting involved in.' My wife is hopeful and skeptical. When I showed them the prototype they all wanted to know how it works and they tried to figure it out."

He says his family can't wait to see the cup, which is made of sturdy aluminum and needs no batteries to operate, make its debut at their seder this year.

"I'm looking forward to seder because it would be the first time to have it in action in its own context," said Jaffe, who is expecting "laughs from adults and amazement from kids."

But once the trick is exposed, will the cup be able to hold guests' interests?

"It will still maintain its novelty, since it is just used for seder," he said. "You won't be showing it off to everybody. You gotta hold off."

Jaffe jokes that the Elijah Drinks cup isn't the end of his Pesach fun. He says he might look into creating an afikoman detector.

"I could [also] do a burning bush and some of the bigger miracles like parting the sea anytime," he said with a laugh. "Why learn to swim when you could part the sea?"