You bought your girlfriend a potted cactus for her office. You're explaining why you thought it was a good idea for a gift.
The good thing about it is it doesn't need to be watered that often.
Use this expression when you want to talk about the good points of something:
The good thing about my job is that I get a lot of vacation time.
The good thing about living in a big city is that you can get everything you need within walking distance.
In grammatically correct, formal English, you should include "that" at the beginning of the clause:
The good thing about it is that it doesn't need to be watered that often.
In conversational spoken English, people often leave "that" out.
Organize a sentence this way when you don't want to say who did something. For example, you might use this to avoid directly telling the listener that they have to do some work:
You know, I think the trash needs to be taken out.
The files in this folder all need to be reviewed.
Sentences like these which use "be (done)" are called "passive" sentences.
When you give a plant water, it's called "watering" the plant.
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