NCERT Solutions for Class 5th Maths Chapter 10 Lali's firki, and what happens when you turn it — Let Us Make a Windmill Firki
Book page 137 Updated on2026-09-19
Q1.
Lali makes firkis for her friends. Follow the steps given below to make your own firki. 1. Take a square paper. 2. Fold the paper in half diagonally to make two triangles. 3. Open and fold it the other way to make two more triangles. 4. Open it again. You will see an ‘X’ shape on the paper. 5. Use scissors to cut along the four lines of the ‘X’. Stop cutting about halfway to the centre. 6. Take one corner of each triangle and fold it gently towards the centre of the paper. Do not press it flat. 7. Fold every other corner towards the centre. 8. Push a pin through the folded corners and the centre of the paper. 9. Push the pin through a stick or straw.
Answer
You need a square paper, scissors, a pin and a stick or straw. Work slowly at step 5 — that is where most firkis go wrong.
Take a square paper. A square is needed so that all four blades come out the same size.
Fold it corner to corner. You get two triangles. Open it out.
Fold it corner to corner the other way. You get two more triangles. Open it out.
Look at the paper. The two folds cross in the middle and make an X. The point where they cross is the centre.
Cut along the four lines of the X — but stop about halfway to the centre. Do not cut all the way, or the paper will fall into four pieces.
Take one corner of each cut triangle and bend it gently to the centre. Hold it there. Do not press the fold flat, or the blade will not catch the wind.
Do this for every other corner — one corner from each of the four blades, going the same way round.
Push a pin through the four folded corners and through the centre of the paper.
Push the pin into a stick or straw. Your firki is ready.
Two diagonal folds, four half-cuts, four corners pinned to the centre.
Why the corners go to the centre: Each folded corner makes a slanting blade. Wind pushing on a slanting blade pushes it sideways, and that side-push turns the firki round the pin.
Tip: Always fold the same corner of every blade — all four to the left, or all four to the right. If you mix them, the blades push against each other and the firki will not spin.
Q2.
Make sure the pin is not too tight. Check if your windmill spins when the wind is blowing.
Answer
Hold the firki up in the breeze, or run with it. If the pin is loose enough, it spins freely.
Test the pin first. Flick one blade with your finger. The firki should go round several times on its own.
If it stops at once, the pin is too tight. Pull it back a little so the paper can turn against the stick.
If it wobbles and falls off, the pin is too loose. Push it in a little more.
Now take it outside, or hold it in front of a fan, and watch it spin.
Why the pin must be loose: The paper has to turn while the stick stays still. If the pin squeezes the paper against the stick, the two are locked together and nothing can turn.
Sample answer: My firki did not spin at first. I loosened the pin a little and blew on it. Then it spun easily. In the school ground the wind made it spin without my blowing at all.
Q3.
Observe the dot in the firki. Does the firki look the same after 1/4, 1/2, 3/4, and a full turn? ___________________.
Answer
Yes. The firki looks exactly the same after a 1/4 turn, a 1/2 turn, a 3/4 turn and a full turn.
The green dot is only there to help you see that the firki really has moved. The dot travels round, but the firki itself looks unchanged.
The blades sit in the same four places every quarter turn, so the firki looks unchanged.
The firki has 4 blades, all the same, spaced equally. One full turn shared among 4 blades = 1 ÷ 4 = 1/4 turn So it looks the same at 1/4, 1/2, 3/4 and full turn — 4 times in one full turn
Why it happens: After a 1/4 turn, blade 1 sits exactly where blade 2 was, blade 2 where blade 3 was, and so on. All the blades look alike, so you cannot tell that anything moved. This is called rotational symmetry.
Remember the book's line: "A firki has rotational symmetry, as it looks the same when rotated by 1/4, 1/2 and 3/4 turn."