NCERT Solutions for Class 4th Maths Chapter 1 Triangles and rectangles made with straws and clay — Shapes with Straws
Book page 14 Updated on2026-09-19
Q1.
Make a triangle with straws of different sizes and clay/plasticine. Does the shape of the triangle change if we gently push one of its sides? Yes/No
A triangle made of three straws joined with lumps of clay.
Answer
No. The triangle does not change its shape.
Join three straws end to end with lumps of clay to make a triangle.
Hold one corner and push one side gently.
The whole triangle moves, but its shape stays exactly the same.
Why it happens: Once the three straws are fixed, there is only
one way they can meet. To change the shape, a straw would have to get longer or shorter,
and a straw cannot do that. This is why the triangle is the most rigid
shape.
Did you know? Builders use triangles inside bridges, roofs and
towers for exactly this reason — they do not bend out of shape.
Q2.
What kinds of angles does a triangle have?
A triangle made of three straws joined with lumps of clay.
Answer
It depends on the triangle you make, but there is one rule that always holds.
A triangle can have three acute angles — like the triangle made of
three straws of the same length.
A triangle can have one right angle and two acute angles.
A triangle can have one obtuse angle and two acute angles.
The rule: a triangle always has at least two acute
angles. It can never have two right angles, and never two obtuse angles.
Why it happens: A triangle has only three corners. If two of
them were right angles or obtuse angles, the two sides going up from them would never
meet, so the triangle would not close.
Check it yourself: Try to make a triangle with two square
corners using straws. The two slanting straws run side by side and never meet.
Q3.
Make a rectangular shape with straws and clay. What kinds of angles do you see in the rectangle?..................
The straw rectangle in the book, and the shape it becomes when one side is pushed.
Answer
All four angles of the rectangle are right angles.
Put the corner of your folded-paper checker on each corner of the straw
rectangle.
It fits exactly at all four corners.
So the rectangle has 4 right angles and no acute or obtuse angle.
Why it happens: In a rectangle the opposite sides are equal and
each side stands square to the next one, so every corner is a square corner — a right
angle.
Q4.
Does the shape of the rectangle change if we gently push one of its sides? Yes/No
The straw rectangle in the book, and the shape it becomes when one side is pushed.
Answer
Yes. The rectangle leans over and becomes a slanting shape.
The four straws stay exactly as long as before.
The opposite sides stay equal and still point the same way as each other.
But the shape tips over, like a gate blown by the wind.
Why it happens: A four-sided frame is not rigid. Its corners can
open and close while the sides keep their length, so the frame can lean.
Try this: Now add one more straw across from corner to opposite
corner. The frame becomes two triangles and it stops leaning at once.
Q5.
What has happened to the angles of the new shape? Are they still right angles? What types of angles have been formed?
The straw rectangle in the book, and the shape it becomes when one side is pushed.
Answer
No, they are no longer right angles.
Two opposite corners have closed up — they are now
acute angles.
The other two opposite corners have opened out — they are now
obtuse angles.
So the new shape has 2 acute angles and 2 obtuse angles.
Why it happens: When the frame leans, whatever one corner loses
the next corner gains. So as two corners get smaller, the other two get bigger by the
same amount.
Q6.
Similarly, push one side of a square. Are they still right angles? What types of angles have been formed?
The straw rectangle in the book, and the shape it becomes when one side is pushed.
Answer
No. The square also stops having right angles.
All four straws are still of the same length, so all four sides stay equal.
But the shape leans, exactly as the rectangle did.
Two opposite corners become acute and the other two become obtuse.
The leaning shape has 4 equal sides but no right angle — it looks like a squashed
square.
Why it happens: A square is a rectangle with all four sides
equal. Since a four-sided frame is not rigid, the square leans over in the very same
way.
Q7.
How are the angles of triangles and rectangles similar or different?
The straw rectangle in the book, and the shape it becomes when one side is pushed.
Answer
Triangle
Rectangle
Number of angles
3
4
Kinds of angle
at least 2 acute; the third may be acute, right or obtuse
all 4 are right angles
What happens on pushing a side
nothing — the angles do not change
two angles become acute and two become obtuse
Rigid?
yes, the most rigid shape
no, it leans over
Similar: in both shapes every angle is made where two straight sides
meet, and in both the angles can be tested with the same paper checker.
Different: a triangle’s angles are fixed for ever; a rectangle’s
angles change the moment you push a side.
Why it happens: Three sides can meet in only one way, so a
triangle’s angles are locked. Four sides can meet in many ways, so a
rectangle’s angles are free to change.