By Steve Gimbel

April Fools' Day is unique among our holidays because it is the only one for which ingenuity is required. Other festivals are celebrated with expectations of prepackaged icons. Try serving something other than turkey for Thanksgiving and you are likely to face a revolt. Holidays are bastions of repetition, but April Fools' Day is a commemoration of creativity.

There is something comforting about tradition. It is undeniably valuable to pass along to your children or grandchildren religious, cultural, or family rituals. Customary practices can bring deep significance.

On the other hand, we often reduce our celebrations to a set of expected symbols, acts, and practices. Valentine's Day is accompanied by heart-shaped boxes of chocolates and cards of pink and red. We give the gifts we think we are expected to give. And so it is with our other holidays. We know what we are supposed to do and buy, and that's just what we do.

Even giving Christmas and Hanukkah presents has lost its creative edge with the rise of gift cards. We worry that we will give the "bad gift." When our loved ones open our offering, we want them to get something they want. So why not let them pick it out? That way, they are sure to get exactly what they want. But something is lost in that transaction. The mind of the giver is not really present in the gift.

But this is not the case with the April Fools' joke. When you get people good, they know they were gotten, and gotten by someone else - usually they know it was you. They are sure that there was an active mind behind the prank, carefully planning its execution.

Think about how a joke works. Most jokes have two parts - a setup and a punch line. The setup leads you to think of a situation in a particular way, to give you a particular view of some element of the world or a created situation. The punch line then shocks you into suddenly having to reinterpret what you thought you knew. Consider Rodney Dangerfield's classic: "My psychiatrist told me I'm crazy. I told him I wanted a second opinion, so he said, 'OK, you're ugly, too.' " When Rodney says he wants a second opinion, we think we know what he means, but then - zing - we have to re-understand what we thought we understood.

The humor is found in that moment when we "get" the joke. Our brains are caught between the competing interpretations because we so strongly bought into the setup, but eventually we realize we have to surrender our old understanding of things for the new way. We get it, and find the incongruity funny, once we can cognitively step back from it.

The same is true of the sort of practical jokes we play on April 1. For the joke to work, the mark has to initially believe something about the state of the world that turns out not to be so. Maybe it is that the world is as we expect it. The door will open if I turn the knob. But then the world is revealed to be different from what's expected. The knob is not attached to the door and comes off in my hand. Or maybe we are confronted with the world in a state worrisomely different from what's normal - say, you pick up your cellphone to find the screen is cracked, but it turns out to just be a sticker.

To design a successful joke, you need to be able to see the same situation in multiple ways. When you are gotten, you are forced to see the same situation in multiple ways. In either direction, this is healthy.

Whether it is the news sources we seek out, the company we keep, or even the food we eat, humans are creatures of habit. We can get stuck inside our own way of seeing, our constrained way of being. But for one special day, we are reminded that we need to open our minds and recognize that things could be understood in completely different ways. That is a good thing, and . . . oh, your shoe is untied.

Steve Gimbel is the Edwin T. and Cynthia Shearer Johnson chair for distinguished teaching in the humanities at Gettysburg College. sgimbel@gettysburg.edu