Sphenoid: of, relating to, or being a winged compound bone of the base
of the cranium
I found out we cannot say “this alien is very sphenoid” (this alien has lots of sphenoid bones).
Rainy: marked by, abounding with, or bringing rain
We can say “today is very rainy” (it rains a lot).
I cannot see any difference.
Best Answer
I think that you have misunderstood the meaning of very. Here is the entry from the Cambridge Dictionary:
If you want to say "a lot of", you do not use very: you use "a lot of" in informal conversation, or many (for countable things) or much (for uncountable things)... and many/much/a lot of must be followed by a noun.
Deciding whether an adjective is gradable is not an exact science. There is no complete list of ungradable adjectives, dictionaries do not carry this information, and, as this Ngram graph shows, usage has changed over the past 200 years. In 1850, it was perfectly acceptable to say "very perfect" or "very excellent", whereas those expressions are not regarded as acceptable in modern English.
It is possible that our perception of gradeability has changed, but it is more likely that using very with a non-gradable adjective was considered reasonable then, but is now considered over-dramatic.
As a general guide, an adjective is considered ungradable if something is either X or not-X. For example, true is ungradable- something is either true or it's not. You will still hear people say "very true", though.
On the other hand, with an adjective like cold, there are varying degrees of coldness, and so it is gradable.
Adjective for parts of the body are definitely ungradable: something is dorsal if it relates to the back, and it's not dorsal if it doesn't relate to the back. There is no in-between.