Scientists are not perfect.  In fact, they may make just as many mistakes as you do at school!  However, they keep going back to make sure that they do not make the same mistake again.  One of the biggest mistakes scientists can make has to do with the conservation of mass, which says that the amount of stuff in the world always stays the same.  This means when they do a test, they have to keep track of every last little atom that they cannot even see.  It is not easy.

Everything around you is made of stuff.  Your books, your eyes, the light coming off of this screen.  Wave your hand through the air.  That's also stuff!  Mass is how much stuff, or matter, is in one thing.  Something can have a lot of stuff packed in tightly, like in a rock.  Something else can have very little stuff, with lots of space in between, like in a sponge.  Mass means the number of atoms that are in something and how close they are to each other.  This is a matter of counting.  Easy, right?  No mistakes so far.

OK, who wants to count the holes in this sponge with me!

Mass is different from something's weight.  A sponge and a rock may take up the same amount of space, but that does not mean that they weigh the same.  Weight means how much gravity is pulling on something.  The best way to remember the difference is that the weight of something can change while its mass will stay the same (so long as it all stays together).  If you take a very big rock up into space, you would be able to lift it because gravity is no longer pulling down on it as much.  The next time you want to show off, take someone into space and show them how you can lift a heavy rock.  They will be very amazed that you could take them into space.  Still no mistakes?  Good.

Count this guy in, he totally rocks!

Remember when you waved your hand through the air?  You feel something pushing back, right?  That's because air has mass and weight.  There are atoms floating all around you.  Even though they are very light, they are being pulled to Earth by gravity.  A gas is a bunch of atoms that are very spread out and have lots of energy and move all over the place.  Gases can bounce around and go in and out of different things all the time.  Plants will gain a very small amount of mass and lose a very small amount of mass because they take in and let out gas.  What you thought was staying just the same can be changing.  Looks like it's time for some mistakes.

The hard thing about gases is that they are very hard to track when scientists do tests.  When watching something react, they might think that the weight that goes into the test will be the same as what comes out.  This does not always happen!  You could start with a solid but get a gas and a liquid at the end, or it could be the other way around!  In tests, things freeze or melt and some parts can be lost as a gas.  This will make what you end up with weigh less than what you started with.  Human error is what can go wrong when people make small mistakes while doing tests.  Scientists can do other things besides forget about gases leaving the thing they're working on.  They can do little things like forget to weigh the glass that something is in or drop a hair into the test.  All the time when people test things, these little mistakes can change everything.

Wait, now did YOU remember to count the weight of the glass?

Ever wondered why scientists will do the same tests over and over and over again?  Mistakes.  That's why.  When thinking about so many other things, they might thing that the weight and mass of something will stay the same when it comes out the other side of the test.  They will not think about the gases that can get away or that little bit of peanut butter that fell out of someone's beard, changing everything.  Often, one of the biggest things we have to look out for in our science tests . . . is ourselves.


Ducksters.  "Mass and Weight"  Ducksters, 2010.  <>  "The Science of Human Error"  Education, 2011.  <>