If x is denoted by a baby, then derivative of x is 1 which is growth but 1 cannot be a developed x. So, we need a formula whose derivative is greater than it. What about derivative of x^2. It's 2x. Now if value of x is 3. Then, 9 grows to 6 which is again hostile in a sense. Growth is not a discrete entity though it occurs to discrete entities. Neither is growth continuous. Growth stops after an age. So probably 9 grows to 6 after middle age. If x is 2 , then 4 grows to 4 maybe for an adult. Now the question is what is the x for a kid.