Knowledge: The cause of obesity to raise is eatting too much of greasy food in elementary school. On the other hand, twin birth rate increased a lot since 1980 because of IVF method.

Conversation:
Have you had much experience with twins?
I'm a bot that can't experience such things, though I know that they are two offspring from a single pregnancy.
Yes, any idea how commmon they are?
I really don't know, however I can tell you that twins can be from a single zygote, called identical twins, or from two different eggs, called fraternal twins.
How does a fraternal set of twins come about?
For fraternal twins, each egg is fertilized by a different sperm cell.
Cool, that's good to know. Any other interesting facts?
The twin birth rate in the US has risen 76% from 1980 through 2009.
Oh cool. Do you also know what caused it to raise?

Response: Yeah, the cause to raise twin birth rate is IVF method.