Below in an excerpt from parenting.com. Read more here!
With my 3-year-old twins in tow, I navigated several steep flights of
 subway stairs, managed four train transfers and arrived safely at the 
Museum of Natural History in Manhattan. I took photos of them in front 
of the giant Apatosaurus skeleton and imparted (probably erroneous, but 
who cares?) facts about the Jurassic era. I am the best. Mother. Ever!
To
 top off the special day, I decided to treat them to an educational toy 
at the gift shop. My son Theo wanted an astronaut, so I brought him to 
the space display and let him choose between three astronaut-themed 
items (I'm so smart to give my preschooler a sense of control by offering him a choice!).
 “No, astronaut!” he began to whine. “This is an astronaut,” I said 
brightly, pointing to one of the helmeted play figures. “No!” He then 
slapped all the items out of my hand and began screaming. Ten minutes 
later, after Theo had stomped on a dozen packages of freeze-dried ice 
cream, I tucked one boy under each arm and staggered out. I am the worst mother ever, I said to myself, embarrassed, drained and near tears.
Turns
 out, the scene at the museum was not all my fault, and it doesn't mean 
my boy is “bad,” either. Michael Potegal, Ph.D., a pediatric 
neuropsychologist at the University of Minnesota, in Minneapolis, has 
spent the latest part of his professional career studying tantrums and 
how and why young children have such brutally emotional explosions. And 
what has he learned in that time? That their outbursts are as normal a 
biological response to anger and frustration as a yawn is to fatigue. So
 normal, in fact, that you can make a science out of the progression of a
 tantrum and predict one down to the second. Kids from about 18 months 
to 4 years are simply hardwired to misbehave, he says. And that means 
“nurture” (i.e., you) isn't always to blame.