Preview · the opening missionRead this to the class:
A Grade 7 student read that her body is made of about 30 trillion cells. She held up her hand and squinted. She couldn't see a single one. "If they're that small," she asked, "how does anybody know they're there? And how small is small?" Your team's job: measure the smallest things you can, then work out how far down a cell is.
Teams of three, with rotating jobs: Measurer (handles the ruler and lens), Checker (repeats each measurement), Recorder (writes and does the arithmetic).
Look inside the PDF
Sample pages, stamped “preview”. The download is the clean, full lesson.
In the full lesson PDF
- Predict (5–10 min)
- Hunt (20 min)
- Reveal (10–15 min)
- Act (10 min)
- Background for the teacher
- Assessment
- Student sheet
On the student sheet
- My prediction
- Our measurements
- Draw what you saw
- Our scale ladder
- Claim, Evidence, Reasoning
- Exit ticket
US Letter, prints cleanly in black and white. The student sheet is on its own page, ready to photocopy.
Free with an account (any 3 lessons), or US$19 / C$25 for all 4 lessons in this unit.
What students show by the end
- Measures small things
- Places a cell on the scale
- Explains with CER
From the lesson’s rubric, which scores each one as not yet, getting there or got it.
Questions teachers ask
How long does “How big is a cell?” take?
50–60 min. Group size: teams of 3.
How much prep does it need?
15 minutes: cut an onion into wedges and separate the layers. Write the scale-ladder cards (listed in step 5) on index cards, one set per team. If you have microscopes, set them to the lowest power. If you don't, print a photo of onion skin cells with a scale bar from a textbook or a reputable science website.
What materials do I need?
Ruler with millimetre marks (a clear plastic one if you have microscopes); magnifying glass or hand lens; table salt; sewing thread and a pencil; onion, cut into layers by the teacher; index cards for the scale ladder; and 3 more. Cost: about $5 for a class of 30.
Which curriculum standards does it cover?
NGSS (US): MS-LS1-1. It also matches Ontario Science and Technology, Grade 8.
What’s in the lesson PDF?
A teacher guide that walks you through predict, hunt, reveal, act, background for the teacher and assessment, then a student sheet (“My prediction”; “Our measurements”; “Draw what you saw”; “Our scale ladder”; “Claim, Evidence, Reasoning” and “Exit ticket”) on its own page, ready to photocopy. It’s US Letter and prints cleanly in black and white.
Can I try it for free?
Yes. A free account includes any 3 full lessons of your choice, and this can be one of them. After that, this unit is US$19 (C$25) for all 4 lessons, yours to keep, or every lesson is US$79 (C$109) a year.