About a year after I bought my MacBook, I noticed that its touchpad no longer clicked when pressed, as if there was dirt underneath it to prevent it from going down.
Later I realized that my MacBook's battery would die almost immediately after it is unplugged. Since I always used it with the power adapter connected, I did not really care. But I did not make a connection between the touchpad problem with the battery.
Another year later, I found that the battery was swollen to the point that the back cover could not be closed completely anymore. So I decided to take it out and replace it.
To my surprise, the touchpad started to click again when pressed while the battery is out. After replacing the battery with a new one, the touchpad still clicks when pressed. Apparently when the touchpad stopped clicking when pressed, it was when the battery started to swell up.
So now I have a perfectly working MacBook, and I now I know that the touchpad can be used as a battery health monitor as well: when it no longer clicks when pressed, chances are that your battery has gone bad and started to swell up.