Watching “Juno” is like opening your mailbox to find it overflowing with boxes of your favorite candy. It is sweet, it is thoughtful, and it fills you to the brim with happiness that has less to do with glucose than with the surprise of having received it.

Yet Jason Reitman’s “Juno” is not really about candy; it is about teenage pregnancy. Juno MacGuff (Ellen Page), the perky 16-year-old heroine, decides that she will have the baby and then give it up for adoption. The emotional baggage of her decision and the moral debate around abortion are not the focus of the movie’s plot. The movie is more concerned with following the process of Juno’s growth, which runs parallel to the growth of the baby within her.

Juno is witty and smart; even though she is not in the most amusing of situations, she never loses the spirit to spout funny lines. The movie’s comedic elements are centered around the endless sauciness that exudes from Juno herself. “They call me the cautionary whale,” she says, and it is hard not to laugh. Page, who is nominated for an Academy Award, is great at conveying this jauntiness of Juno with natural grace.

While Juno can outsmart anyone in witticisms, she is still very much a child. She wears checkered Converse shoes with orange laces, calls the abortion clinic from her hamburger phone and is more fascinated by gory movies than by the ultrasounds of her baby. Growing up is a central theme in “Juno,” and Juno gets her fair share of it. She shows her naiveté in thinking she has found the ideal happy couple to adopt her baby: the Lorings, whose life appears to have sprung out of ‘Pleasantville.” Yet through her evolving relationship with Mark and Vanessa Loring (Jason Bateman and Jennifer Garner), Juno acquires a more mature perspective on the realities of love and marriage.

The story is conventional, but the movie derives its force from unpretentious delivery, a sense of humor and genuinely likable characters. These include Juno’s father (J. K. Simmons) and stepmother (Allison Janney), who manage to pull off the we’re-not-thrilled-but-we’re-supportive-parents. Still, they are almost too ready to accept Juno’s pregnancy, and the we’re-not-thrilled part is less emphasized than the we’re-supportive part. Juno gets off easy, but perhaps the movie wants us to believe that this is what it means to be understanding and gentle parents.

Paulie Bleeker (Michael Cera), the father of the baby, is a colorful presence in the movie. His appearance in crimson and yellow running gear marks the beginning of each new season, which corresponds to each trimester of Juno’s pregnancy. The passing of time is also depicted through Bleeker’s team running in occasional blurs of crimson and gold in the background. Bright lollipop colors are pervasive throughout the movie, complementing indie folk tunes that add to the flippant mood. Only in the suburban house of the Lorings do somber colors take over — colors that have names like “custard” and “cheesecake.”

When Juno explains to Bleeker why she put a hundred packs of orange tic-tacs in his mailbox, she says “I figured you could never have enough of your favorite one-calorie breath mints.” That’s simple enough, but it’s true. It is also true that the happiness “Juno” creates is even more lasting than that of your favorite candy.