Feeling guilty about that giant ice cream sundae you enjoyed at your niece's birthday party? Don't beat yourself up! It takes a lot of calories—3,500—to gain a pound of body fat. "So really, that one off day doesn't usually result in any significant weight gain," says Newgent. It's about what you do the next day and the day after that's really important—so don't stay off-track. So be sure to whittle away at those extra calories over the next day or two, preferably by boosting exercise rather than eating too little. Starvation is not the healthy answer!