Close the Loop
A bulb lights only when current can leave the battery, go through it and come back: a complete loop, not just a connection.
A battery, a bulb and some wire are laid out on a grid, with gaps left in the wire. You put pieces of wire into the gaps. The bulb lights the moment the loop is finished, and not one moment before: a wire that stops in mid-air does nothing at all, however close it gets.
The second circuit looks finished but the bulb is dark. There are three gaps and you get one piece of wire. Two of those gaps sit on wires that lead nowhere, so only one of them can bring the current home.
Then two bulbs, both lit, and a harder job: put the right-hand one out while leaving the left one lit, and without taking away any wire that touches it. It can be done, and when you have done it that bulb is still joined to the battery at both ends. Being joined to the battery is not what lights a bulb. Being on the loop is.