Plants have cycles of making food and using food. Plants breathe in CO2 during the day and use it to make food which they store. At night, they are like animals, breathing in oxygen to use their food and breathing out CO2.

If your plants are not growing in size, it is shedding carbon at the same rate it is taking it in and they won't need any extra CO2.

There isn't much carbon dioxide in the air, only about 0.04% by volume. Your plants aren't going to grow unless they can get more CO2. They can probably get a little more CO2 from microscopic leaks in the tank.

You must realize that green plants also produce some carbon dioxide of their own. Photosynthesis is a two part cyclic process. Carbon dioxide will also be produced as organic matter decomposes in the soil within the terrarium, thus this will also be used by the green plants and oxygen released in the exchange during the process.

That having said, there are still reasons to think that no one can really stop the continued warming of the world. Not even the Kyoto Protocol – an already massive initiative, to say the least, taken by humanity to address the problem – dares to dream of averting the rise of world temperatures all together. As indeed, our optimum technologies nowadays have yet to frame adequate solutions to curb the rate of global warming. If this perceived helplessness in respect to global warming speaks of anything about our present situation, it merely proves that there is more into the problem than merely attributing it to human doing. This is because if global warming is really a problem constituted by human fault, then it is with human initiatives that such problem can and must be remedied. Apparently however, human progress cannot be blamed for the recent rising trends of global temperatures.

Global Warming as the Earth's Natural Way of Being

If you want how, you should read about photosynthesis

So I did a class project on terrariums and I put some little young plants into a glass jar with soil, pebbles, sand, and finally added some water, then closed the lid. If, supposedly, the lid is air tight, then do the plants survive for long without much carbon dioxide? Because just like us humans, we will die when there's no oxygen in our brains for a few minutes.

I understand how water is recycled. The plants breathe, then the water vapour/droplets become water and slowly drip down onto the plants again, and vise versa. You don't have to explain that to me. I just don't understand how air is recycled.