*Add water to a cup on a scale to see if increases in weight. If it does (which we think is super likely, duh!), they we can claim a plant gets its matter from water. If it doesn't increase, then it's fair to claim water doesn't provide plants with the matter it uses to grow.
***Spolier: It increased in weight! Water is made of matter!
*Weigh a balloon before and after you blow it up. It its weight increases, then air is made of matter, and therefore a plant gets its matter from air, too! If it doesn't increase, then air isn't made of matter and plants don't use air to grow taller and new body parts, and therefore, increase in weight.
***Spolier: Air is made of matter. The weight of the balloon went up by just a little, but that increase still tells us air is made of matter!
*We were on a roll! If using a scale to prove that water and air were made of matter, then the same investigation would have to work for light, too! We made it super dark to have no light on the scale, and then we shined a lot of light on the scale to see if light was made of matter, too.
***Spoiler: No matter how much light, or even the type of light (artificial vs. natural)...light didn't weigh anything. Light isn't made of matter and therefore, even though it's needed to help a plant grow, it's not where a plant gets its matter from!