What's the pattern with losing/gaining weight?

I keep on going on random diets and stuff but they don't help me lose weight. My friend's natually skinny and I often follow her (I eat what she eats). She eats icecream and I follow her. I noticed that I wasn't getting fat. I'm really confused about...

this weight thing, how does it exactly work?

It's not a good habit to get into...losing and gaining weight.
If all she eats is a little ice cream once a day, then yeah obviously you're not going to get fat. And if all you eat is ice cream, then you're probably getting enough calories to where you're not gaining weight. If you're not eating tons and tons of calories, you're not going to gain any weight.

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You have a BMI of 19.9.

BMI is between 18.5 and 24.9 (Normal Weight)

People whose BMI is within 18.5 to 24.9 possess the ideal amount of body weight, associated with living longest, the lowest incidence of serious ilness, as well as being perceived as more physically attractive people than persons with BMI in higher or lower ranges. However, it may be a good idea to check your Waist Circumference and keep it within the recommended limits.

BMI (Body Mass Index) is a measurement of body fat based on height and weight that applies to both men and women between the ages of 18 and 65 years.

BMI Classification

18.5 or less Underweight
18.5 to 24.99 Normal Weight
25 to 29.99 Overweight
30 to 34.99 Obesity (Class 1)
35 to 39.99 Obesity (Class 2)
40 or greater Morbid Obesity

Check out your BMI at the bmi calculator site below to see where you stand.

bmi calculator:

http://www.bmi-calculator.net/

If you need conversion info, 1 kg = 2.2 pounds and 2.54 cm = 1 inch (12 inches = 1 foot)

Info if you are trying to GAIN weight:

http://www.bmi-calculator.net/treatment/underweight-treatment.php

Info if you are trying to LOSE weight:

http://www.bmi-calculator.net/treatment/overweight-treatment.php

How can I Lose 1 pound of body weight per week?

One pound of body fat is equal to about 3500 calories. So to lose 1 pound of weight per week, you need to eat 500 calories/day LESS than you burn.

But it's not a good idea to lose weight simply by cutting calories. As well as decreasing calories you should increase your daily physical exercise. This helps to burn calories and build more lean tissue which in turn burns more calories.

What should a minimum calorie intake be?

As a guide to minimum calorie intake, the American College of Sports Medicine (ACSM) recommends that calorie levels never drop below 1200 calories per day for women or 1800 calories per day for men. Even these calorie levels are quite low.

http://www.bmi-calculator.net/bmr-calculator/harris-benedict-equation/calorie-intake-to-lose-weight.php

Normal Adult Height-Weight Range Table

http://www.recipehound.com/weight.html

Average Teenager Weight

http://lifestyle.iloveindia.com/lounge/average-teenager-weight-236.html