SSS: three sidelengths fix a triangle completely

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What to assume they know

Same shape and same size: superimposition as the test — congruence, superimposition, and the licence to turn or flip a figure before testing it
Drawing a circle of a given radius about a given point with a compass
Reading a length off a ruler and a measuring tape
A triangle has three sides and three angles, and knowing some of them may or may not pin down the rest
The idea of a fold line: a crease about which two halves land on each other

What they should be able to do

State the SSS condition in the form the chapter states it
Carry out the segment-and-two-circles construction for three given lengths
Explain why that construction yields two points, not one
Give at least two ways of testing whether the two resulting triangles match
Argue that the two are congruent using the base as a fold line
Explain why the argument does not depend on the particular lengths used
Say what SSS lets you do that measuring the angles as well would not add
Recognise a common side as a third equality you never had to measure

Where it usually goes wrong

"Two circles crossing twice means the triangle is not determined." This is Rabia's own worry and it is a good one — it is exactly what sinks SSA on p.12. What rescues SSS is that here the two crossings sit on opposite sides of AB, so the two triangles are each other's flip. The middle step is the lesson.
"You need the angles as well, to be safe." Meera's line is the chapter's position: with the three lengths in hand, measuring the angles adds nothing. Extra measurements cannot make a determined figure more determined.
"SSS is true because I drew three triangles and they matched." Three drawings are evidence. The reason is the fold-line argument, which uses no particular numbers at all — and that is why swapping 4, 6, 8 for 40, 60, 80 changes nothing.
"A side shared by both triangles cannot count." It is the most useful equality in the chapter. In Fig. 1.1 the shared segment BD is what turns two given equalities into the three SSS needs, and the same move reappears in every later exercise built on a crossing or a shared edge.
"Any three lengths make a triangle." The chapter does not raise this. Say only what the chapter says: given a pair of triangles whose three sidelengths agree, congruence follows.
"Congruent means the two triangles are sitting the same way up." ΔABE and ΔABF on p.5 are congruent while pointing in opposite directions. That picture is the cure.

Questions to check understanding

Given three lengths, construct the triangle and state why any classmate's drawing must be congruent to yours
Given two triangles with all three pairs of sides marked equal, state the condition and express the congruence
Given a figure in which two triangles share a side, list the three equalities that establish SSS
Explain why the two triangles produced by the two circle crossings are congruent
Decide whether a stated set of three equalities is enough, and name the missing one if it is not
Show that a segment splits an angle into two equal parts by finding congruent triangles either side of it (the shape of Figure it Out question 3, Part II, p.9)
The SSS bullet of the SUMMARY (Part II, p.22) is the compressed form the chapter itself expects back

Examples worth working on the board

The frame (Part II, §1.2, p.4). Checked against the printed page. A colour drawing of two girls beside a wooden triangular frame standing on the grass, roughly chest-high on them; the taller girl holds a measuring tape with a clipboard tucked under her arm, and the other holds a protractor. The frame is not huge — what stops them tracing it is that it is bigger than a sheet of paper, which is what the printed text says.
The measured frame (Part II, §1.2, p.5). Inputs: the three sides come out 40 cm, 60 cm and 80 cm. Rabia reaches for a protractor and Meera stops her.
The scaled-down version (Part II, §1.2, p.5). Inputs: 4 cm, 6 cm, 8 cm — chosen so the triangle fits on a page.
Rabia's construction (Part II, §1.2, p.5). Checked against the printed page. AB is drawn horizontally and tagged 6 cm. Two dashed circles are drawn — the smaller about A, the larger about B — and they cross at two points, E above the line and F below it. Both ΔABE and ΔABF are drawn in. Inputs: base 6 cm, radii 4 cm and 8 cm from its two ends. Let it produce the two triangles itself.
Three tests (Part II, §1.2, p.6). The chapter offers three ways to settle whether ΔABE and ΔABF match: trace one and compare, cut one out and lay it on the other, or notice that AB behaves as a line of symmetry because the same construction was performed above and below it. The third is the one that generalises; the first two are the ones a student can do at a desk. Show all three, and say which is the reason.
The result (Part II, §1.2, p.6). Equal sidelengths force congruence. The chapter sets the statement on its own line and then gives it the name SSS.
The rectangle (Part II, §1.2, Fig. 1.1, p.7). Checked against the printed page. Rectangle ABCD with A at the top left, B at the top right, C at the bottom right and D at the bottom left, and one segment drawn from B to D. Inputs: AB = CD and AD = CB because it is a rectangle. The third equality — that BD belongs to both triangles — is the one worth stopping on. It is a length nobody measured, and it completes SSS for ΔABD and ΔCDB.
Figure it Out, question 3 (Part II, §1.2, p.9). Checked against the printed page. A four-cornered figure with A at the top, B at the left, C at the bottom and D at the right; AC is drawn as a dashed segment across it. Tick marks show AB = AD (single) and CB = CD (double). Inputs: those two pairs, plus AC shared.
Figure it Out, question 4 (Part II, §1.2, p.9). Re-checked against p.9. The outline is a dart, not a triangle with a point on its base: D at the apex, F at the lower left and G at the lower right, and E between them but sitting above the F–G level, so the shape reads D–F–E–G with a notch at E. No segment FG is drawn, and E does not lie on one. DE runs vertically from the apex down to the notch. Ticks: DF and DG single, FE and GE double. Inputs: DF = DG, FE = GE, DE common. Note: the two triangles here are congruent by SSS, but the pairing the question writes down is not the pairing that works — that is the point of the question, and it belongs with Naming congruent figures so the correspondence is unambiguous.

Figures to have open

The segment-and-two-circles construction, redrawn: a horizontal base, two arcs or full circles from its ends, and both crossing points marked and joined. Standard schematic, but it must be able to be shown moving — the two crossings have to appear one after the other.
A fold or reflection movement about the base line. This is the argument, not a decoration.
Rectangle ABCD lettered exactly as Fig. 1.1 letters it (A top left, B top right, C bottom right, D bottom left) with the segment BD drawn. Standard schematic.
A triangular frame standing on the ground, chest-high on the person beside it and wider than a sheet of paper, for section 1. Redraw; the book's own illustration should not be lifted.
No photograph or data table from the textbook is needed.

Where this sits in the book

NCERT Ganita Prakash, Class 7, Part II, printed Chapter 1 "Geometric Twins", §1.2 "Congruence of Triangles", pp.4–6 — the frame (p.4), the unnumbered subheading "Measuring the Sidelengths" with Meera's claim and Rabia's construction (p.5), and the three tests, the fold-line argument and the naming of SSS (p.6)
Same part, same chapter, §1.2, "Conventions to Express Congruence", Fig. 1.1 and the surrounding rectangle argument, pp.7–8
Same part, same chapter, §1.2, "Figure it Out", questions 3 and 4, p.9
Same part, same chapter, SUMMARY, p.22, third bullet
Forward pointer: SAS, and why the angle has to be the included one, where the same two-crossings picture produces the opposite verdict for SSA