Z and I had a conversation about 911 earlier today - what it used for, why it is a short number, what types of situations she might need to call 911 for and what she should say if she did, and "how much" someone would be bleeding if they cut themselves so bad that an ambulance would be necessary. Thus the amount of blood in the human body arose as a significant question.
We did a quick web search and came up with a figure of six quarts of blood in an average adult. (Later, and perhaps better figures would knock this down closer to five quarts, but I have no regrets.) This means very little to a five-year-old; even showing a one-quart liquid measuring cup and saying "six of these" doesn't help much, because kids that young can't really visualize and estimate volumes. So we decided to make six quarts of "blood," to get a sense of just how much that was.
It all went down in the bathroom, where knew we could clean up any mess we made without much hassle.

We used this hair clip board to count quarts as we went. With each quart we poured into the larger container, Z "opened the doors" in one of the hair clip slots.

The red food coloring was the master stroke that made certain Z would both remember and understand what we were doing. Above, Z contemplates the volume of blood that is supposedly coursing through my body.
The next step was a discussion of the fact that she undoubtedly had considerably less blood. We did some quick math and guessed that although she was about 1/4 of my weight, she might have up to 1/3 of the amount of blood in her body that I had in mine. We really had no reason to think that, but it seemed like a decent guess, and I didn't want to slow down our progress by looking for more facts.
That's when things got really interesting, though, because after calculating how much blood we were going to measure back out of the container, we decided we'd take pictures of each of us with the quantity of blood that was in our bodies. In other words, it meant Z was going to get her hands on our
Nikon D40. She's a pretty careful kid, but that camera is pretty heavy for a five-year-old.

In a way it seems like a lot of blood to have in your body... doesn't it?
When we removed the blood from the container to "draw down" to Z's level, she "closed doors" back up in our counting card and was able to tell me when we had removed enough.

This seems like a shockingly small quantity of blood for anyone to have, especially since we were probably overestimating. It's realizations like this that make kids' toughness in the face of great vulnerability so shocking, and so inspiring.

This photo is a few months old, from the time Z ran out of a restaurant more or less straight into a pole. There was a lot of blood at the time, and it didn't help that her tears mixed with it, reopening it several times as it healed. Clearly, though, there was plenty of blood left over.