This is a cell from a young mouse, spreading on a glass slide and observed by fluorescence microscopy at 630 times magnification. It is very small. Its diameter is perhaps 20 micrometers, or about 0.0008 inches. If you can't picture that, think about taking an inch and splitting it into two equal pieces, then keep doing that 9 more times. It's small. Superman would have trouble seeing this. Yet, there's a whole world inside.

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We only see three components of this world here. The red is "actin", a protein that forms very dynamic "filaments" - rods that can be used in all sorts of ways: as pathways between two parts of the cell, fingers to push or pull on things and move them around, ropes to hold things together, or belts to tighten around something and even to squeeze that something in two. The actin filaments are made up of little actin "monomers" which act sort of like bricks in the filament. Each monomer is about 64 billion times smaller than the cell. Most cells in our body have over 100,000,000 actin monomers. We have lots of cells, 100,000,000,000 neurons alone. The actin filaments don't last long - often they form, do their specific job, then disappear in a matter of seconds. That's because cells are always moving in one way or another, and nothing can be too permanent in them. In the second it took to read this sentence several trillion actin monomers either added to or fell off filaments all over your body, helping you stay ready for the next thing coming. Each time an actin adds to a filament, it "burns" energy in the form of adenosine triphosphate (ATP). In the time it took you to read this page, over a mole of ATP (602,000,000,000,000,000,000,000 molecules) were "burned" by actin in your body. Life is a very active process. We see in this cell a number of thick actin fibers reaching out in all directions. They are helping the cell sense the environment, looking for a clue as to where it should go.

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The green is revealing "microtubules", which make different filaments from actin but equally transient, forming then vanishing in seconds as they fulfill specific tasks. They are actually tubes, much thicker than actin and hollow in the center. In some respects, microtubules do similar jobs to actin - they can be used as paths for things or fingers to push or pull on things. But being thicker and hollow, their properties are quite different, so their specific tasks are different. Another tool in the cell's toolbox. Here, the microtubules are all tangled up in the center, because the cell's all Higgsledy Piggsledy, having been just plopped into a new environment a few minutes ago. We have about as many building blocks for these microtubules as we do for actin. In other words, a lot.

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The blue is the DNA "genome" - the codebook that allows most things in the cell to be made again and again. Each cell has a lot of DNA. Three billion letters of code in the codebook. If all the characters were typed out into an actual book, that book would have about a million pages. Now that's a good book. Apart from being the code for everything, DNA's a bit boring because it doesn't do anything else. We have to take care of it, though, because it's the code.

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We only catch one instant in the life of this cell here. About a second. We can guess, though, at what it's doing. It's seeking. Spreading out to explore a new environment. Actin filaments reaching out in all directions, testing the waters to see which path might be best. Microtubules waiting to lay down roads as the cell finds its direction. DNA being de-coded to provide more actin, microtubules and many other things to keep the cell going. This cell has made contact with another cell to the left, and is soon to make contact with the cells in the upper and lower right.

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But there are all sorts of things we don't know from this snapshot of the cell's life. We don't know where it came from or where it's going. Will it decide to keep all its connections with the other cells? Will it choose some but not others, enriching and elaborating those connections it decides to keep? Will it head off in its own direction, breaking away and going out into the void? This type of cell likes to be with other cells. The void's scary, black and cold. But sometimes a cell can wander off into it, in search of new things. Confident that it will find a place it likes, so it can stop moving, make attachments. And grow.