Tips! Eat every 2 hours. Drink juices, soft drinks or milk like all the time, especially when your at home. Carry nuts in your bag so you always have some protein to snack on wherever you go. Avoid sugary foods cus they get used up faster in your stomach. EXERCISE. Guys: Go to the gym at least 4 times a week. Girls: The same, otherwise jogging everyday early morning works. This is important so you don't develop little fatty areas around your body, like an old person....dripping...of fat....eugh....
Because you're not eating over your maintenance.
Your body requires a certain amount of calories to maintain it's current
weight (body weight in lbs x 15)...so if you were 150 pounds, that would be
150x15 = 2,250 calories/day.
It is also possible you are eating a bit more than your maintenance but because you are physically active (running/or engaging in some other sport) you are burning those extra calories (1 hour of strenuous activity = about 500 calories burned).
Also, keep in mind, not all foods are created equal.
You could eat a pound of broccoli and only add 154 calories to your
daily caloric intake, or you could eat a pound of pizza and add 1,200
calories to your daily intake.
You could be eating a lot of low calorie foods throughout the day.
Also, one last thing is that your idea of 'a lot' may not be in line with other people's ideas of what a lot of food is...and you may in fact not be eating much food at all.
Regardless, if you started counting every single calorie that went into your mouth, got a food scale and weighed out ALL your food...and also track your calories burned through exercise and even walking...you'd quickly determine what is going on.
I wouldn't recommend weight gain as it can cause acne