Be forewarned: You may be smiling at a crocodile — and at an alligator — this Sunday at the South Shore Natural Science Center. Herpetologist Michael Ralbovsky of the Beverly-based Rainforest Reptile Shows will present “Alligators and Crocodiles” at 11 a.m. and 1 p.m. “This show is great because most people don’t know the differences between alligators and crocodiles,” Tracey Merrill, the center’s executive director, said in a phone interview. “Michael does a great job of explaining the differences, like how the reptiles are hatched, what their habitat is, their diet . . . it‘s educational and fun, and kids love that they get to actually touch them.” Tickets may be purchased at the door or in advance by calling 781-659-2559. Tickets are $7 for members, $10 for non-members, and include admission to EcoZone, the center’s interactive museum.