Here's a fun little science demonstration that gets lots of "Ooooohs" and "Aaaahs" from the class.
When you teach about the human body, or about the five senses, discuss the human eye. Teach kids that the black part in the center of your eye is the pupil.
Explain that the pupil changes sizes. Your eye needs just the right amout of light. When it's too bright, your pupil gets smaller so less light can get in. When it's very dark your pupil gets larger so more light can enter your eye.
Have each student pair up and face the person next to them. Tell them to watch their friend's pupil (black part of the eye) closely.
Pull down the shades and flip off the lights in the classroom. Students will see their friend's pupil immediately increase in size.
After a few moments have passed, and you've discussed why the eyes' pupils increased in size, have them again watch their friend's pupil. Flip on the lights and the kids can watch their friend's pupil quickly shrink in size.
Kids love this demonstration in all elementary grades!!