3 plastic cups
A dark closet and a window sill that gets some sunlight
Plant all three cups of grass seed using the same amount of soil and seeds. Water them throughout the experiment as needed. Place two of the cups in a dark closet and one on the window's edge for one week. (I did this the week before my photosynthesis lesson so that I could show the kids the results immediately.) After one week, observe the differences in the vigor of the plants growing in the two different locations. Move one of the plants from the closet to the window. (I spend two days teaching photosynthesis, so it worked out to be a Friday and a Monday. It worked out perfectly to reveal the plant experiment on Friday and discuss how it relates to photosynthesis and then make some predictions about how the plant being moved might change over the weekend. Checking the plants was a great springboard into Monday's lesson.)
Update! Looking for an INB printable to go along with this? You might like...