Nemo: I was reading this biology lab supply catalog to help me fall asleep, and I found this weird looking microscope slide. It’s called a blood-cell-counter.

L. J. Silver: Are ye sure ya weren’t reading yer zombie supply catalog again?

Nemo: Oh… umm…maybe you prefer the Latin name: hemocytometer.

L. J. Silver: So what’s so special about it?

Nemo: Well, for one thing, it has the ability to spread a drop of water out into a perfect box-like shape.

L. J. Silver: Oh yeah, that’s something I’ve often wanted to do…

Nemo: No, no, really, it's hard to subsample a drop of water because it's curved on the top and rounded on the edges. And I still have a social life.

L. J. Silver: Arr, ok, perfect box-like shape, yo ho.  I guess that could make things easier.  Still, $150 for a microscope slide seems, you know, kinda pricey.

Nemo: Well, it also has lines etched into the glass to make the counting easier. 

L. J. Silver: Really?  I don’t see any lines.

Nemo: That’s because they’re really small.  You can only see them under the microscope.  I found a picture on google – here, have a look…

L. J. Silver: Arr, ok, so I see lines – too many lines!!  What in Davy Jones locker am I supposed to do with all those lines?  This thing is just too confusing.  I say we go back to counting the old-fashioned way – make the land-lubbing interns do it all!

Nemo: Sigh… Really, sir, if you would give me a moment to explain… Remember how we had big fish and small fish?  When we wanted to count big fish, we used a big grid, right?  And to count small fish, we used a small grid. 

L. J. Silver: Arrr...

Nemo: Well, bacteria and algae and cells and all that microscopic stuff comes in different sizes too!  So sometimes we need a big grid, and sometimes we need a small grid.

L. J. Silver: So you’re saying we need to buy lots of these pox-infested blood-cell-counters, one for each size?  This just gets worse and worse.

Nemo: No, sir, that’s the point.  You only need to buy one – that’s why there are so many lines.  There is a large grid, and nested inside that there’s a medium size grid, and nested inside that is a small grid.

L. J. Silver: So which grid do we use?

Nemo: The one that’s just right, of course!