I show her how to use a ruler, balance scale, and measuring cups, because I want her to understand the importance of getting data for yourself, and because I want her to grasp the connection between counting numbers and dimensional measurements.
I bought her a laboratory stand to hang strings and pulleys from. She's fascinated by this. I explain that when things move against one another, there's something called "friction" that makes them not want to move. She wants to set up the string so that it can move things at a distance and in a different direction, and I show her how the friction is stopping it from moving, and how she can use a pulley to make it move easier.
Mostly I let her play by herself and discover things, but I like to point things out so that she'll notice them: when the stand starts to tip over, I show her how the force she's putting on it is making it act like a lever, and I show her how she can move the base so that it doesn't tip over.
"It's like magic!" she says.
"It's science," I tell her. "It's like magic, only it works better."