Line and ray: what changes when you refuse to stop

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What the video covers, what to say before it, where a class usually goes wrong, and what to set afterwards. An account is free, and it opens every chapter of every book.

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What to assume they know

A point fixes a location; a segment is the shortest route between two points: point, line segment, end points, and the convention of naming points with capital letters
The idea of extending a drawn stroke further in the same direction
Every number sequence is a rule, not a list: a rule can describe something that never terminates

What they should be able to do

Describe a line as a segment extended without end in both directions
Explain why no drawing of a line is ever complete, and what the arrowheads stand for
Write a line three ways — through two named points, or by a single small letter
State and justify that two points determine exactly one line
Say how many lines pass through a single given point
Define a ray by its starting point and its one endless direction
Name a ray correctly, putting the starting point first
Explain why two different second letters can name the same ray, while swapping the two letters does not
Read a mixed figure and list its points, lines, rays and segments separately

Where it usually goes wrong

"A line is a long segment." Length is exactly what a line does not have. Corrected by asking the student to name a line's end points and watching the question fail.
"You could draw the whole line if you had a big enough sheet." Corrected in §2.3 by the book's own question — the answer it gives is no, and the reason is that endlessness is in the definition, not in the paper.
"Ray OA and ray AO are the same, because segment AB and segment BA are." This is the trap the previous topic set up. Corrected in §2.4, p.17: the two spellings name rays with different starting points, so they run opposite ways.
"A ray must be named by its far end." Corrected by Fig. 2.7 — any point lying further along the ray serves as the second letter, so one ray has many legitimate names, all beginning with the same letter.
"Through one point there is one line, just as through two points." Corrected by Rihan's question in §2.4, p.15: through a single point the number is unlimited, and the second point is precisely what cuts it down to one.
"Arrowheads mean the same as 'this is a straight line'." They mean continues without end, which is why a segment drawn straight carries none.

Questions to check understanding

Name the rays shown in a figure and state, for each, whether a nominated point is its starting point
Say how many lines pass through one given point, and how many through two
Draw a rough figure to a written specification — two rays meeting at a named point; two lines intersecting at a named point; a line containing two named points but not a third
Given a ray drawn through several points, list its alternative names and reject the illegitimate ones with a reason
From one mixed figure, list points, a line, rays and segments under separate headings
Explain in one line why a line cannot be measured

Examples worth working on the board

Fig. 2.2 (§2.3, p.14). A single stroke through A and B with an arrowhead at each end, and the small letter m set beside it — the same object named two ways in one picture.
Notation, three forms (§2.3, p.14 and Summary, p.54). Segment: two capitals under a plain bar. Line: two capitals under a bar with an arrowhead at each end. Ray: two capitals under a bar with one arrowhead.
Rihan and Sheetal (§2.4, p.15). Rihan marks one point and asks how many lines pass through it; Sheetal marks two and asks the same. The answers run in opposite directions — through one point, endlessly many; through two, exactly one. Present them as a pair, because the contrast is the content.
Fig. 2.5 (§2.4, p.16). Vertex T with one path running up to the right through A and another running right through N and then B. The rays present are TA, TB, TN and NB. Three of those four begin at T; the fourth begins at N instead, which is why it is spelled NB. This is the cleanest test of whether a student is reading the starting point from the figure or from the alphabet.
Fig. 2.7 (§2.4, p.17). One ray leaving O and passing through B and then A. It may equally be called OA or OB, since B lies along it; it may not be called AO, because that names a ray starting at A. Two questions that look alike and answer differently — keep them adjacent.
Fig. 2.6 (§2.4, p.16). A mixed figure: a straight path with arrowheads at both ends carrying D, E, O and B; a further arrow leaving O upwards; and a further arrow leaving O to the right through C. Ask for five points, a line, four rays and five segments. Note that several correct answers exist for the last three parts — the exercise rewards a defensible reading, not a unique one.

Figures to have open

Fig. 2.2 — one stroke, two arrowheads, points A and B, and the letter m. Standard schematic.
Fig. 2.3 — a ray with its starting point A and a further point P on it, one arrowhead. Standard schematic.
Fig. 2.5 — the two-ray figure at T with A on one path and N, B on the other. The positions of N and B along the second path carry the whole question, so keep their order.
Fig. 2.6 — the mixed figure at O described above. Needed as drawn; a simplified version stops the exercise from working.
Fig. 2.7 — one ray from O through B to A. Needed as drawn, because the order of B and A along the ray is the point.
The lighthouse, torch and sun images may be redrawn freely; they are illustrative, not load-bearing.

Where this sits in the book

NCERT Class 6 Mathematics (Ganita Prakash), Chapter 2 "Lines and Angles": §2.3 Line, p.14; §2.4 Ray, pp.15–17
Figure it Out items used: §2.4, Q1 and Q2 on p.15–16, Q3 and Q5 on p.16, Q6 on p.17
Summary, p.54, for the settled notation of line and ray
Backward pointer: §2.2 Line Segment, p.14, which this topic extends