Experiments...

WV: What are your students talking about today?
ME: Umm... we're talking about experiments. Do you know what that is?
WV: No.
ME: Umm... an experiment is like when you see if something sinks or floats. It's when you ask a question and see what happens. 

[We visit my teaching lab and feed carrots to sea urchins.]

On way back to his school:
WV: So today your students are going to see if things sink or float.
ME: Oh, no, we're not talking about that. We're talking about experiments with animals.
WV: So you're going to see which animals sink or float?
ME: (chuckling) No, we're going to talk about experiments to see how animals grow. You know how animals start out tiny and then they grow and change?
WV: Yeah. Because they get lots of food.
ME: Yes.