We know how to weigh the amount of water in our mini-lakes, but how could we weigh the water in a raindrop? Introduce today's investigation question:

What does a drop of water weigh?

Give students a chance to share ideas. Challenge students to help refine the question:

• Are all drops the same size?
• What size drop of water do we weigh?
• Can we control the size of a drop?
• Can we rely on the classroom scale to provide the answer?

Acknowledge that drops can be different sizes, and explain that the class has a tool-a dropper-that makes a drop of a certain size.

Show students the dropper. The class will address today's investigation question by determining the weight of a drop from this dropper.

Revise the investigation question to read:

What does a drop of water from our classroom dropper weigh?

Post the rephrased investigation question.