Michelle Stepp of Nashville first noticed it when her son, Keaton, was about 3 months old. "When he was excited, he would just shake all over," she recalls. "We thought, 'Wow, what is he doing?'"

Just being a normal baby, it turns out. "Some babies will shudder or shiver involuntarily when they see someone they recognize or a toy they want," says Kristin Concepcion, M.D., a pediatrician in Fort Lee, NJ. There's no one reason: Some just express themselves that way, others may have immature nervous systems. "As long as your baby is developing well and meeting all of his milestones, it's probably fine."

There are times, however, when it is worth a doctor visit. If the shaking lasts more than 20 seconds, if the baby loses consciousness, if his eyes flick from side to side, or if his lips turn blue, call your doctor immediately, says Dr. Concepcion. "These could be signs of a seizure or a neurological disorder."

Keaton is now 2 1/2. "He still shakes occasionally," says Stepp. "But now that he has more words, he can just say that he thinks watermelon is yummy."