When a girl falls and gets the slightest itty-bitty scratch, she will cry for 5 minutes, then she will be upset again later when said scratch is exposed to bath water. A boy falls down a set of brick stairs, scratches his leg and knee up to the point of bleeding, then gets up, catches his breath, and gets back to playing (ie: torturing his older sister) without a single shed tear.
A girl will back down, cry, and continuously apologize when you begin to sternly correct her. A boy will spit, shout "NO!", run away from you, and sometimes bite if you attempt to infringe on his "fun".
Balls, motorcycles, tractors, airplanes, and all things motorized are the center of a little boy's universe. Barbies, dress-up clothes, tea sets, make-up, and doll houses are what a girl chooses to spend her time with.
A boy will eat just about anything you put in front of his face, and he'll need to eat quite often to curb any meltdowns. A girl will be picky just like her momma 9 out of 10 days, and then totally surprise you when she cleans a chicken drumstick down to the bone and then asks for seconds.
Clothes are important to a girl as she wants to change them multiple times during a day. A boy would rather run around with his belly button hanging out, but only after he has covered every inch of his clothing in dirt or food or who knows what.
A little girl gives love and kisses to her mommy, daddy, and brother. A little boy shows the same sweet affection. :)
DISCLAIMER #1: This list may or may not be indicative of your experience with boys and/or girls. I can only vouch for my two examples. List is subject to adjustment in a few months. We'll have to wait until then to find out which gender will outnumber the Castle house and to put these rules to the test!
DISCLAIMER #2: There are exceptions to every rule.....