What You Need:
- Clear plastic cup
- Medicine dropper
- Pitcher of water
- Blank paper for recording
- Pencil or marker
- Clean towel
What You Do:
- Place the cup on a flat surface. Fill the cup to the very top with water.
- Help your child to make a recording sheet with the piece of blank paper. At the top of the piece of paper, help your child make a heading for the page … “How many drops of water will a full cup of water hold?” Draw a two-column table on the piece of paper. Label one column “My Prediction” and label the other column “The Result”.
- Ask your child to describe the cup of water. Ask her if she thinks the cup can hold more water. Then ask her to make a prediction (guess) about how many drops of water it will take to make the cup overflow. Allow your child to record her prediction.
- Begin the experiment. Give your child a medicine dropper and allow her to add water drops to the full cup of water. Make sure that your child counts EACH drop she adds to the cup. You may want to use paper and pencil to make tally marks to keep track of each drop of water added to the cup. Encourage your child to watch the water “stretch” as she adds drops of water to the already full cup.
- How many drops of water did it take to make the cup overflow? Allow your child to record the number of drops of water it took to make the cup of water overflow on the recording sheet in the result column.
- Once the experiment is over, discuss the results with your child.
- Try exploring the same concept by adding drops of water to the head of a penny. Observe to see if your child uses the experience with the cup of water in making a prediction about the number of drops of water a penny will hold. (Your child may have predicted a small number for the first experiment, but she may use what she learned from the first experiment to make a prediction about the number of drops of water a penny can hold.)
What's Going On?
Water and other liquids have a thin film that covers their surface. Forces within the water (surface tension) pull the water molecules closer together and make the water seem to have a skin. The “skin” created by surface tension is able to stretch so that water poured into a cup will look as though it is stretching beyond the top of the cup.