Provide students with a cylinder of water (without lines, words, etc. on it - check your recycling bin for empty glass bottles) and an index card with an arrow printed on it.
Have the students observe the arrow, looking through the cylinder of water. What happens as the move the arrow away from the water?
Provide students with plenty of time to play with these simple objects, allowing them to make as many observations as possible. Some ideas to try out:
- watching the arrow through the water while the rotate the paper
- holding the arrow vertically instead of horizontally
- observing other shapes or letters through the lens