Can anyone think of a good science misconception and how to go about
dispelling it for primary children.

a eavy object falls faster than a light one.

My kids think north is up...as in, towards the sky.

As for dispelling this notion, let me know if you find a way.

Paf, but they're right.  Well oriented kids; wouldn't have expected
otherwise   of yours.  :)
Tell them about the zenith and nadir (had to look it up, sounds like Star Wars).

That the earth is flat; that the sun and moon "rise"; that the moon is
eaten by that Indian demon whose name I have forgotten; why there
cannot be a pot of gold at the end of the rainbow; that dogs and cats
and salt and pepper are not some kind of natural opposites  (as I once
thought - too much pepper, add salt);  ...
There are lots of misconceptions about where babies come from, but
that might not be part of your program.
Geometric conceptions:  That it is not shorter to walk home from one
corner of a block by walking towards home (then being closer) and then
walking along the block to the diagonaly opposite corner of the block
for the rest of the way home, compared with walking around the block
the other way (had an argument with Jane in the third grade about
this).
That the longer water boils, the hotter it gets.

As a small child, I used to think that you boiled eggs to make them
soft. (Well, it's true for meat, carrots, potatoes, etc.)

Or there is the myth that cream is heavier than milk (so why does it
float to the top of the bottle?).

Right, Amber, the misconception that a thicker liquid must be heavier
than a thinner one (my childish terms: thicker and thinner).
Neatsie experiment of making different layers of liquids in a glass.

More misconceptions:

There is no gravity in space.

Infrared radiation is a form of heat energy.

There is a dark side of the moon.

Electric current follows the path of least resistance.

Pafalafa-ga, since you asked:
It suddenly occurred to me this morning that youngsters who grow up in
front of a monitor  - or experience certain images only on a monitor -
 may see some things through this "window" differently than older
people do.
You and I know that a map is a horizontal paper representation of
geographical features and understand that on a vertical monitor we are
looking at electronic representation of this.  But kids, possibly, see
a map on the screen as a vertical thing, without this understanding: 
North is up. (full stop!)  That is what they see, and every map image
is consistant about having an N-arrow pointing up.  (I hadn't thought
about this with my first flippant comment.)

It is rather like the American natives who could not conceive of ships
appearing on the horizon and described them as moving mountains.

Does that make sense? (IF it does, it could raise other interesting
questions about computer/internet-wise kids' perceptions of many
things: from not understanding that solitaire is a game played with
little stiff cards; to who-knows-what about perceptions of other
online games, and much in between, besides maps.)

HOw to dispell the "north-up" misconception:  Maybe print a map of
your local area, one on which you can pin-point where you live, and
then take them out and orient it so that they can see that it shows
the street and next corners, and then with a compass prove that north
on the map image and compass north are in one horizontal direction,
and maybe that the setting sun is to the west (but at sunset in June
it will be further north, so that may detract from the lesson.

(As a preschooler, I had some feeling that they had to cut the heads
off people to put their voices on the radio, although I knew this
could not really be so, but I remember still looking for a scar when
my sister returned from a broadcast about girl scouts.)

Have good weekend, Myoarin

Myoarin,

An interesting thought. It must surely be true that watching TV and
surfing the Internet alter a child's perspective, but most of the maps
I remember from grade school were hanging on a wall, so again North
and Up were the same direction, though I don't recall having any
confusion about this. Also globes are usually oriented with North up.
We also look at many photos horizontally, and have no difficulty
making the 90 degree mental rotation. So I don't think TV and the
Internet can be blamed for this.

However, it is difficult for a child to grasp the arbitrariness of
this convention. As a child, I remember being confused about why
people in Australia don't fall off the world, but not so confused
about people in North America. A good way to force a child to confront
the confusion of this arbitray convention, and perhaps dispell it, is
to post in the classroom one of those maps that puts North at the
bottom. See for example:

http://www.flourish.org/upsidedownmap/

On first viewing, a map like this seems completely wrongheaded, but of
course, it is an equally valid way to depict the globe, just contrary
to convention.

Regards,

JWT