What information convinces us that something is a living thing? To address this question, I showed children and adults two types of animations (Opfer, 2002).

•The first type, exemplified in Video A, depicts a blob moving itself, with independent changes in direction and acceleration.

•The second type, exemplified in Video B, depicts the same blob moving the same way, but toward a goal.

If you're like most 5-year-olds and adults, you should think that Video B depicts a living thing (e.g., a jellyfish or a germ) and that Video A is ambiguous (i.e., it looks like a jellyfish but it also looks like an empty bag).

In fact, both agents are neutrophils, a mindless cell that lives in our blood. Neutrophils engage in a type of goal-directed movement called chemotaxis. Chemotaxis keeps neutrophils alive by leading them to food, but it also kills them by leading them to fight off pathogens such as E. coli. To see lots of neutrophils in action, visit Cells Alive!

All living things engage in some form of goal-directed movement. Plants, for example, expend energy to turn toward sunlight, or direct their shoots from the ground, or direct their roots toward water. Teaching children about the goal-directed movements of plants convinces most children that plants, like animals, are living things (Opfer & Sieger, 2004) -- a fact children typically don't realize until age 10 (Opfer & Gelman, 2001).

In addition to goal-directedness, other features of movement may also help people figure out whether something is alive (Opfer & Gelman, 2010). For a whole list of really cool demonstrations (thoughtfully compiled by Brian Scholl), click here.