A battery and one bulb in a loop light the bulb

Why

Deepak is imagining the battery as a source that gives each bulb a fixed, full amount of push no matter how many bulbs there are. But in a single loop, the same current has to pass through both bulbs one after the other, and adding the second bulb makes it harder for current to flow around the loop — so LESS current flows than before, and less current through a bulb means a dimmer glow. On top of that, the battery's push now has to be shared between the two bulbs instead of going into just one. Test it: build the one-bulb loop and note the brightness, then add the second bulb in the same loop — both light up, but noticeably weaker than the single bulb was. His sister is right: two bulbs in one loop each glow dimmer, because the current through the loop drops and the battery's push is split between them.