Materials & Prep
- Per group: clear tub half-filled with water, towel, and these labeled test objects: large untreated wood block, small wood craft stick, small steel bolt, steel washer, plastic bottle cap, and large plastic container lid.
- Copy one Sink-or-Float Investigation Sheet per student with columns: Object, Material, Size (small/large), Prediction, Observation, Evidence for my claim.
- Post the words observation (what I notice) and inference (what I think it means). Draw a class T-chart labeled “floated” and “sank.”
- Confirm water-use, spill-cleanup, and hand-washing procedures against school expectations. Students do not drink the water or splash.
Sink-or-Float Puzzle
0–7 minDisplay the large wood block and small steel bolt without naming their materials. Ask: “Which will sink: the big block, the little bolt, both, or neither? What makes you think so?” Students silently commit to a prediction with a thumbs signal or by recording it. Invite two contrasting reasons, especially “big things sink.”
Have students place the two objects in water and watch without touching. Ask them to state observations only: “The large block stayed on top. The small bolt went to the bottom.” Add results to the T-chart. Confront: “Our prediction that big things sink does not match what happened. What else might matter?” Explain that an observation is what they saw; an inference is an idea about why.
Modeling the Pattern
7–15 minIntroduceTell students they will test the question before making a claim: “Does size decide, or does the material an object is made from give us a better clue?”
Think aloudHold up the small craft stick and the large wood block. “The block is bigger, but both are wood. I predict both will float. I am looking for a pattern in the material, not just one object.” Test both and record: wood, large, floated; wood, small, floated. Then show the small steel washer. “It is small like the craft stick, but it is steel. I predict it will sink.” Test and record the observation.
ExplainSize alone cannot tell us whether an object will sink or float. Materials matter because equal-sized pieces of different materials have different amounts of matter packed into them. Compare two same-size boxes, one filled with cotton balls and one filled with coins: the coin box has more mass for the same space. Wood and many plastics often float in water; solid steel often sinks.
ClarifyDo not change an observation into an inference. “The washer sank” is an observation. “Steel sinks because it has more mass packed into the same-sized space than water” is an inference based on the observation. Shape and trapped air can also affect an object, so today’s claim must be supported by the objects tested.
Check for understandingPoint to the large plastic lid and ask, “It is larger than the steel washer. Does its size prove it will sink?” Students turn and talk using the stem: “No. Size alone does not decide because . I would check the material by .” Listen for students naming a test rather than repeating “heavy things sink.”
Guided Investigation
15–25 minStudents work in groups of four. Assign roles: tester places one object in the water, recorder completes the group sheet, observer watches closely, and reporter prepares an evidence statement. Rotate after three objects.
Have students predict, test, and record the plastic bottle cap, large plastic lid, steel bolt, and steel washer. Pause groups after two tests. Ask: “Find two objects that are different sizes but had the same result. What material were they made of?” Add class results to the T-chart.
ScaffoldGive each group the sentence frame, “The and the are different sizes, but they both . They are both made of . This is evidence that .” Provide picture-and-word labels for wood, steel, and plastic. Students may point to the chart while a partner records.
Look forStudents should use “material” and specific observations. Prompt a student who says “the washer sank because it is heavy” with, “Compare it with the bigger wood block. What does that show about size?”
Evidence Claim Challenge
25–39 minStudents independently choose two pieces of evidence from the class data, then meet with their group to create a three-part claim on their investigation sheet. Their final response must include: a claim about size and material, two observed examples, and reasoning that connects the examples to the claim.
Post this checklist: 1. I named material and size. 2. I used two observations. 3. My examples include objects of different sizes. 4. I explained why size alone is not enough.
Expected“Size alone does not decide whether an object sinks or floats. The large wood block floated, and the small steel bolt sank. Also, the small craft stick and large wood block both floated because they were wood. The material gives a better clue than size.”
CirculateAsk groups to revise claims that say “all plastic floats” or “heavy objects sink.” Require them to use only what their observations support.
ExtensionGive ready students this new situation: “A huge steel ship can float. Does that prove material does not matter?” Challenge them to explain that ships are hollow and shaped to hold air, so the whole ship is not a solid chunk of steel. They should distinguish this special shape-and-air case from the solid steel bolt and washer tested today.
Closing Check
39–45 minExit ticketShow a picture of a large wooden log and a small solid metal marble in water. Students write or dictate: “Which is more likely to float, and why? Use the words material and size.”
Listen for oral shares from two students, then collect tickets. A successful response explains that the larger wood object can float while the smaller metal object can sink because material, not size alone, is the better predictor.