What is the difference between 'stage' and 'grade' when I am referring to the students thar tare in school and in grade 1 or 2. Thank you for helping!

What is the difference between these to sentences; I travel by an airplane. I travel by airplane. I believe the second sentence is incorrect, but I have seen natives use it. Than you for your help!

Are these two sentences correct? 1. The gas rises up. 2. The water starts to come out as a gas.

Which sentence is correct? " Water is liquid." and "Water is a liquid." or " Steam is gas" and " Steam is a gas". Is it necessary to use an article before liquid or solid? Thank you for helping.

Can we use the preposition "in" with the word "bridge"? for example " he is sitting in the bridge." I have always used "on" but I hear someone using "in" and it made me confused, does it mean the same thing as "on the bridge"?

Are these sentences grammatically correct? 1. Fish have scales. ( I know that plural of 'fish' is fish' but still, I am not sure.) 2. I breathe the air. (What confuses me is using 'the' before air.) 3. It is nonliving. ( can we use nonliving alone, or should we add 'thing' after it? Thank you very much for helping.

Can I say, " Desert is hot." or ' Rainbow is colorful."? because I have heard that we can't use nouns alone without an article; except for the zero article when we generalize things. is it the same or different? Thank you for helping!

Can we say" There is pollution on earth."? Thank you for helping!