Multiple slides with covers (plastic preferred)
Onion skins (red and yellow)
Garlic skins
Plant leaves
Assorted pre-made slides
Q-tips or toothpicks

This is a good introductory experiment that ties in well with most botany and biology units.  Generally, if the class periods are shorter than 45 minutes, this experiment will take two weeks.

Begin by explaining how a microscope works.  Explain the different lenses and the different magnifications.  Explain what each of the knobs are used for and how to carefully make adjustments.  Explain the proper care of the microscopes and how to handle them.

How come I don't see anything?
Issue each of the students a slide.  Show the class how the slide fits under the microscope and how to read the slide label.  I found it best to begin by letting the students try and focus with the smallest magnification first with an easy-to-see slide.  (You may have to draw what the students should see on the board or walk around and check on each group individually.)  Once the class is in focus, allow each group to switch to the next higher magnification lens by making minute adjustments to the knob.  When the students have this procedure down pat, it's time to move on.  (This may take longer than you think!)

Wow!  What's this?
Issue each of the students a pre-made slide.  It's best to use things that they would be familiar with, such as blood, insects, different types of fibers, etc.  Once a group has their slide in focus, it's probably best to explain what are the cells and cell parts and what each of the students are looking at.  I found it best to break the students into stations, so that the microscopes aren't being adjusted a whole lot.  It's easier to move the students from station to station than try to get each slide into focus every three minutes.

Let's make our own slides!
Once the students have grasped the technique of focusing in slides and using the microscopes, it's time to let them create their own.  Probably the easiest slide to make involves using the skin of an onion.  The benefit to using an onion skin is that the pieces are already extremely thin and dye is not needed to color the slide.  Have each of the students rip a small section of onion skin and place it on the slide.  Next, have each group place the slide cover on top of the skin.  Place this under the microscope with the clamps holding the slide cover flat.  You should be able to see a pretty good picture of each of the cells.  Repeat with the red onion (it shows up really well), the garlic, and a piece of plant leaf.

Why not make it personal?
After students have looked at the onions and leaves, many will ask to see other things.  One of the best examples turned out to be human cheek cells.  Have each of the students ready a slide and a slide top.  Have the student scrape the inside of his/her cheek with the toothpick of Q-tip (a toothpick works better but may not be appropriate for younger students) and swab the slide.  Apply a small amount of dye and cover.  The cheek cells are pretty large and show up quite well.  The students had fun with this one!

Shawn Len, 2000.