Lesson plans > Density 
Students get confused by the density=mass/volume because most of them haven't mastered ratios. Here's a more intutive approach  do labs to define mass and volume, then show the density=mass/volume formula and do this demo.
1. Principle: If you pack more mass into a the same volume, it's more dense.Common experience: going on vacation, suitcase is too full to close

Common experience: crushing styrofoam (ie. everyone's put teethmarks on a styrofoam
cup sometime or another)
Show a block of styrofoam. Get a hyper volunteer to jump on it until it's crushed
a bit. Then ask the class if it's mass changed. (No, it'd still weigh the same on the scale).
Then ask the class if it's volume changed. (Yes, it's squished, it's smaller now). Is it
more or less dense? (let people hash it out awhile then say, yes, it's more dense, and write
the second principle on the board).
Common experience: styrofoam packing chips, paper clip
Preparation: stuff a big, big plastic bag with styrofoam packing chips until, to you, it
feels obviously heavier than a paper clip. Then get a student volunteer to hold the bag in
one hand, and the paper clip in the other, and say which feels heavier. (If you have a
strong bag, and some time to kill, it's fun to have the students toss the bag around the room
at this point to get an idea of its weight). Now ask the students which is more dense,
the paper clip or the bag of styrofoam chips. (At this point, I got an almost unanimous class vote
for the paper clip since it was obviously metal). Then ask if you could assume something
was more dense just because it had more mass. Write the third principle on the board. Ask how you could tell if some substance was denser than
another substance. Try to avoid saying much here  let them debate it out and ask
questions. (It depends on how much mass is packed into a volume  if you had the same
volume of two things, then the one with more mass would be denser). At this point, the
students ruminated & asked me case scenarios (ex. if I smushed the bag down
to be really, really small  could it be denser than the paper clip).
Summary: After the activity, ask students what they think the word density means. Get back answers and list point form as notes on the overhead. Restate and rewrite the three principles.
Note: this is a nice time to show the formula and integrate with math and ratios now.
Run through the three scenarios above, ask if mass or volume increased, if density
increased. Ask if the numerator or denominator is changing in each scenario and what
effect that has on the ratio. Speculate about
1. what sorts of things could happen in the numerator or denominator to increase a ratio
2. what sorts of things could happen in the numerator or denominator to decrease a ratio
More lesson plans by this author at FunBasedLearning.com