I’m wondering what exactly the difference is between beans, lentils, grains, nuts, and seeds. For example, what’s the botanical difference between black beans, green lentils, rice, almonds, and sunflower seeds? Are they technically all seeds?
Also, what differentiates wheat and oats? They look quite similar. Why are oats often rolled but wheat grains never are?

Lentil Soup Recipe
What is the difference between beans, lentils, grains, nuts, and seeds?
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March 12th, 2010 - 3:01 pm
Beans are seeds produced by a particular group of legumes. The term is not botanically precise and is also applied to unrelated plants such as Cocoa or Castor.
Lentils are the seeds of a specific species of legume, Lens culinaris. There are a number of different types of lentils, but all are cultivated forms of the same species. Also read about pulses, a term covering a wide range of legume seeds including beans and lentils.
Grains are the seeds of grasses, completely unrelated to beans and lentils. Usually this just refers to those plants cultivated for food.
Nuts are a specific type of fruit which develops are very hard coat around just one or two seeds. Just a few tree species produce true nuts. The term is also used more loosely for any large rich seed kernel contained in a hard coat. This includes a wide range of unrelated plants such as Peanuts, Coconuts, and Almonds.
Wheat and Oats are both grains, but different species in the same family. Wheat has been extensively bred into many cultivars from a handful of wild species. Oat is a single species and cultivated forms are still recognisable similar to the wild plant.
Rolled oats are simply the flattened de-hulled seeds. Oats have a tough husk and relatively thick strong bran layer. The husk is discarded and the bran layer remains to hold together each individual oat. Wheat has weaker husks and a thinner bran layer and doesn’t roll into convenient chunks. It is more commonly milled, with or without the bran, into flours or other processed forms such as semolina. The bran is sometimes separated and used for particular foods or animal feed.