There are two types of connections: series and parallel. If there are parallel connections there is always more than one loop (as shown in one circuit here). In this topic you'll deal only with series connections, so with a single loop. Children are expected to develop a more sophisticated model of electrical loops, including parallel loops, somewhat later on.
In the first step an extra battery is added to the circuit. This is compared to the simple circuit studied up to this point.
Then, this same simple circuit is compared to one where there are two bulbs in a single loop.
This is a nugget in the PN thread – connect all three threads from any link
Physics Narrative (PN) Teaching & Learning Issues(TL) Teaching Approaches(TA)