A man stealthily opens a new jar of peanut butter and, using a toothpick, writes a racy note for his wife to find.

In another family, a woman creates a secret code. She's found a way to communicate with her husband when they are at his parents' house.

In another household, each time the parents find themselves in a rough patch in their relationship, they make a reservation at the restaurant where they got engaged. It's their special place. By the end of the dinner, they find they can laugh together again.

Each example comes from the new book, "What Happy Couples Do, The Loving Little Rituals of Romance." The authors, Carol Bruess and Anna Kudak, both teach interpersonal and family communications at the University of St. Thomas in Minnesota.

In the introduction, Bruess and Kudak write, "If you want a long-lasting happy marriage, you should do something about it. But what exactly should you do?" They stop short of advising the reader to copy the couples in their book. The point is to create your own intimacies and rituals, not to copy someone else's.

Playfulness is at the heart of many of the relationships described in "Happy Couples." The couples almost seem like elementary schoolchildren, with their secret codes and naughty notes, their hideouts and their nicknames.

In any relationship, there are probably a million idiosyncracies, notes Lisa Diamond, University of Utah associate professor of psychology. Each unique detail offers an opportunity for a couple to bond with each other in a lighthearted way, she says. "It is impossible to be involved with someone for any length of time without developing a shared history."

The difference between a successful and an unsuccessful relationship is the ability to build on that history. "The trick is, in the midst of busy lives, to try to remember those unique things. To connect."

The details don't seem funny to anyone else but them, Diamond says, recalling some of the things that make her and her partner laugh. There was the time they were traveling and stopped at restaurant in a small town in Wyoming. Each menu item came with no less than four starches as sidedishes — crackers, spaghetti, bread and potatoes.

To this day, whenever they are eating at a less-than-elegant meal, Diamond says, they stop and look at each other and smile. They've had worse.

Couples don't need to think their memories need to be amazingly exciting or hilarious, she says. "As long as it is shared, as long as it contains the moment, 'I know exactly what you are thinking."'

At Brigham Young University's Department of Family Life, professor Stephen Duncan agrees that playfulness is important. But he adds that he would not go so far as to say, "Everything I know about relationships I learned in kindergarten."