Why do the water drops form?
1. Ask the question
![Close-up of the 2-bottle system](../../img/5CFT_FPS_Sp10_I09_05_small.jpg)
The two-bottle systems have been set up and changing for a few days by now. The inside surfaces of both bottles have areas that are covered with different sized drops of water, and a puddle may have started to form in the upper bottle.
Today's investigation question is:
Why do the water drops form?
Before students start to answer the question they'll have another opportunity to observe the bottles.