• Quick retrieval from earlier learning (How fossils form). Students answer on mini whiteboards.
• Show answers together and note who needs support in the main task.

1. In how a fossil forms, what happens straight after "minerals slowly replace the bones over millions of years"? (the rock is worn away and the fossil is uncovered / it is quickly buried by mud or sand / the soft parts rot away / an animal dies) → the rock is worn away and the fossil is uncoveredPrompt: Only the hard parts usually become fossils.
2. Why do most fossils show bones, teeth and shells rather than skin? (Animals had no skin / Skin turns into rock straight away / Scientists remove the skin / Hard parts last longer; soft parts rot quickly) → Hard parts last longer; soft parts rot quicklyPrompt: Which parts of an animal rot first?
3. In how a sea shell becomes a fossil, what happens straight after "a sea creature dies"? (layers of sediment cover it / the fossil is found in the rock millions of years later / its shell sinks to the sea floor / the sediment slowly turns into rock) → its shell sinks to the sea floorPrompt: Fossils form very slowly, buried in rock.

• Pass round different rocks and ask how each one might have formed.
• Share the objective: Rock layers: reading time from the bottom up. Read the success criteria together.

• Work through the example out loud, saying why you do each step, not just what you do.
• Example: At the bottom of a lake, mud builds up about 2 cm every 100 years. A layer of mud is 22 cm thick. About how many years did it take to form?
• Step 1: 22 ÷ 2 = 11
• Step 2: 11 × 100 = 1100
• Step 3: So the answer is: 1100
• Answer: 1100
• Pause after each step and ask a student to say what comes next.

• Do these together. Students try each step on whiteboards before you reveal it.
• Use the hints as prompts rather than giving the answer.

1. At the bottom of a lake, mud builds up about 1 cm every 100 years. A layer of mud is 6 cm thick. About how many years did it take to form? → 600Prompt: How many groups of 1 cm are in 6 cm?
2. A rock layer contains rounded pebbles and fossil river snails. What was the environment when it formed? (a glacier / the deep ocean floor / a desert with no water / a river) → a riverPrompt: What rounds pebbles, and where do these snails live?

• Students work on the student sheet. Circulate and check the first two answers of every student early.
• Students who finish: write a question of their own for a partner, with the answer.

The 6 questions are on the student sheet below, with answers in the key.

• Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.

1. At the bottom of a lake, mud builds up about 5 cm every 100 years. A layer of mud is 20 cm thick. About how many years did it take to form? → 400Prompt: How many groups of 5 cm are in 20 cm?
2. A river has cut a canyon and you can see many layers in the walls. What do the layers show? (one year of weather / the river's colour / paint added by people / a record of how the area changed over a very long time) → a record of how the area changed over a very long timePrompt: Each layer formed in different conditions.

Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-4.