Rice is a staple crop that can be used to make Flour, as well as being part of several recipes. Rice is found in Water Gardens. Rice is a re-harvestable crop, right click the fully grown crop to harvest it safely. Left-clicking will destroy the crop and will NOT return a rice plant.
Prior to version 1.10.2, rice could only be planted in water. The block below the water had to be dirt, and you had to aim at the dirt below the water to plant the rice on top of the water.
In some versions, you have to till the soil and then right click with a shovel to get a different looking tilled dirt. You can then use a bucket of water to cover nine of these special farmlands (3x3 area).
Note: on Space Astronomy... I could not get rice to grow until I placed it in water by clicking on the sand beneath it. no tilling or dirt just sand on bottom and water on top.