By the end of Grade 2:
Identify common objects as solids, liquids, or gases.
Throw plasma in there, the most common state of matter known in the universe. You have common examples in the classroom, the incessant hum of fluorescent lights above. You have great examples outside, the sun and the stars. Some students may have plasma televisions at home.
How many teachers have been tripped up by this "simple" question: what is the sun made of?
If you want to keep a state of matter up your sleeve, save the Bose-Einstein condensate for high school.
By the end of Grade 4:
Objects and substances have properties, such as weight and volume, that can be measured using appropriate tools.
I spent weeks teasing apart weight and mass in a freshman science class. Mass is, at this level anyway, the amount of matter (call it "stuff") in something.
Weight is a measure of the force of gravity on the stuff you are measuring. It depends on where all the other objects in the universe happen to be at that moment, since everything is pulling on everything else.
The closest huge ball of stuff is the Earth, so weight and mass seem synonymous.
They're not--your mass does not change on the moon, but your weight does--but you knew this already. Even 2nd graders know this. You can show them astronauts jumping around the moon and ask them why they can jump so high. They'll parrot the standard answer ("less gravity").
I suspect most of us are afraid to touch gravity because we just plain don't get it.
I would love to start the year with a class full of young adults who get that we don't get it.
That's how science starts.