Baby Math: percentiles

I overheard a comment in Mommy/Baby Pilates class this week that made the math teacher in me want to cry:

"Well, the pediatrician seemed worried - he said our baby is at the 50th percentile for weight. I don't know if we can get her up to 100%, but we're sure going to do our best!"

I was so proud of my husband, who is not-so-much a math geek but understood why this was both sad and funny when I told him about it.

Ok, so just to clarify about percents and percentiles:
percent describes a proportion compared to the number 100. If you took a test and got half the questions right, you would have 50%, whether it meant scoring 10/20, 4/8, or 125/250.

percentile describes a value compared to a normal distribution. If a baby is at 20th %ile in weight, it means that it is *normal* for 20 out of 100 babies to weigh the same or less at that age. A baby whose weight is at the 50th %ile is exactly at the population average for babies that age. And a mother who succeeds in fattening her child up the 100th %ile now has a kid who is heavier than *every* other baby that age!