Classroom activity Grade 4 Science

Design a Filter for Pinegrove's Cloudy Water: A Fair-Test Build

A grade 4 build activity where teams design a layered gravity filter within a layer limit and a time limit, then test it.

Grade: Grade 4
Subject: Science
Material: Classroom activity
Time: 45 min
Prep: Moderate prep
Materials: 1 clear 2-liter bottle with the bottom cut off (teacher pre-cut), per group, 2 cups coarse gravel per group, 2 cups sand per group, 1 cotton pad or cloth square per group, 1 coffee filter per group, 2 clear plastic cups per group (one to hold muddy water, one to catch filtered water), 2 cups of muddy water per group (2 tablespoons soil mixed into each cup of water, prepared before class), A stopwatch, A 0-10 cloudiness scale card, A team data sheet for before and after scores and drain time

Standards

3-5-ETS1-1 NGSS

Define a simple design problem reflecting a need or a want that includes specified criteria for success and constraints on materials, time, or cost.

How this material aligns: The design brief states Pinegrove's filter problem with a specific success measure (cloudiness score cut at least in half) and constraints (a 4-layer limit and a 3-minute time limit), the criteria and constraints this standard requires students to define.

All materials for 3-5-ETS1-1 →

3-5-ETS1-3 NGSS

Plan and carry out fair tests in which variables are controlled and failure points are considered to identify aspects of a model or prototype that can be improved.

How this material aligns: Step 6's procedure holds the amount of muddy water and the pour rate constant across every team so only the filter's layers differ, the controlled fair test this standard requires to find what can be improved.

All materials for 3-5-ETS1-3 →

Download Word file

This material is in the public domain (CC0) and may be copied, edited, and shared freely, without attribution. Updated 2026-07-16.

Objectives

Students will define criteria and constraints for a water filter, including a layer limit and a time limit.
Students will build and test a layered filter, measuring cloudiness before and after filtration.
Students will explain why gravel, sand, and cloth layers trap different sizes of particles from water.

Vocabulary

Filtration: Filtration is the step where water passes through layers of sand, gravel, or other materials that trap the small particles still left in it. It works like a very fine strainer.

Also: criteria, constraint, sediment

Teacher overview

This is a small-group build activity for teams of three or four, set in the fictional town of Pinegrove, whose pond turns cloudy and muddy after every storm. Teams are given a design problem with stated criteria and constraints: build a layered gravity filter using an upside-down cut bottle that cuts the water's cloudiness score by at least half, using no more than 4 material layers and finishing the pour within 3 minutes. The activity maps directly onto the target standards, since teams must define their design against those exact criteria and constraints before building, then run a controlled test to see whether their design actually meets them.

Activity Directions

Turn the pre-cut 2-liter bottle upside down and set it into the empty catch-cup so it works as a funnel.
As a team, choose up to 4 layers from the material list (gravel, sand, cotton or cloth, coffee filter) and decide their order from top, where water enters, to bottom, where filtered water exits.
Place the coffee filter or cloth layer at the very bottom of the funnel neck first, so loose gravel and sand above it cannot fall straight through.
Add each chosen layer in order inside the funnel, coarsest material (gravel) on top and finest material (sand, then cloth) toward the bottom, staying within the 4-layer limit.
Rate the muddy water's cloudiness using the 0-10 scale card before pouring, and record the "before" score on the team data sheet.
Fair test: Start the stopwatch and slowly pour the full 2 cups of muddy water into the top of the filter at a steady rate, matching the same pour rate and total amount every other team in the class is using. Stop the stopwatch when the last drop passes through, and record the time in seconds.
Rate the filtered water's cloudiness using the same 0-10 scale card and record the "after" score. Do not taste or drink the muddy or filtered water, and wash hands after cleanup.
Check the design against the class's stated success measure and constraints: did the cloudiness score drop by at least half, did the filter use 4 layers or fewer, and did it finish draining within 3 minutes (180 seconds)?

The activity ends once every team has recorded a before score, an after score, a drain time, and a check against all three constraints. Gather the class for a whole-group comparison once every team has finished.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key

Debrief question: Why did the coffee filter or cloth layer need to go at the bottom of the funnel neck instead of the top? Answer: To hold the loose layers in place.
Without a bottom layer to hold everything, the loose gravel and sand above it would fall straight through the funnel opening instead of trapping sediment as water passes through them.
Debrief question: One team's filter finished draining in 40 seconds but barely changed the cloudiness score. A second team's filter took 170 seconds but cut the cloudiness score in half. Which team met the class's success measure, and why? Answer: The second team.
Meeting the design brief required both staying within the 180-second time limit and cutting the cloudiness score by at least half; the second team's filter met both parts, while the first team's fast filter did not meet the cloudiness requirement.
Debrief question: Real treatment plants filter water in layers similar to this model, but they add one more step before the water is safe to drink. What is that step, and why is it still needed after filtering? Answer: Disinfection.
Real plants add a disinfecting treatment, such as chlorine, to kill germs that filtering alone does not remove, which is why this classroom-filtered water should never be treated as safe to drink.

Differentiation

Support (below grade level)

Provide a suggested layer order (gravel, then sand, then cloth) as a starting point, and let the team decide only whether to add or skip the coffee filter layer, rather than designing the full order from scratch.

Extension (above grade level)

Ask teams to redesign their filter using only 2 layers instead of 4, predict whether the cloudiness score will drop by more or less than their original design, and run a second fair test to check their prediction.

ELL supports

Demonstrate steps 1 through 4 in front of the whole class before teams begin, labeling gravel, sand, cloth, and funnel on the physical materials as each is introduced. Allow teams to record their before and after scores using the number scale card alone, pointing to a score rather than writing a full sentence.

Extensions

Ask students to sketch a labeled cross-section of their team's winning filter design and write one sentence explaining why that particular layer order worked better than the others tested in class.