system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does the press to test feature work on AA batteries?

On some AA batteries, you can squeeze two points on the battery and it indicates how much juice is left. How does that work?
gpt
They use a combination of colour changing and conductive ink. 

The ink is conductive so that when you touch the two points, current can flow. 

The colour change occurs with temperature. 

So what you do is create a gradually widening strip of the colour changing conductive ink. The narrow part takes the least power to warm up and change colour, and as it gets wider, more power is needed. So it's a way of creating a 'fuel gauge' for the power that's left in the battery. The more of the bar that fills up, the more charge the battery has.