Essay and Assay are easily confused words.
The spell-check application of most word processing software programs would not catch a slip-up of these two words. Spell-check is looking for words that aren’t in its dictionary, and words that resemble words in its dictionary but are possibly spelled wrong. Spell-check isn’t perfect. It doesn’t know and can’t guess what word you wanted or what word you meant, it can only judge the words on the page. If you used words that are all spelled correctly, it gives you a pass anyway.
Autocorrect suggests words that start with the same letters. It suggests what word you may want to save time, but quite often, its suggestions couldn’t be more off base and produces humorous results.
Essay is a noun. An essay is a long written composition on a particular idea or theme. Students often have to write essays in school about summer vacations, required reading, and later, how they see a particular problem in life or the world.
Assay is a verb. It means to analyze or survey the contents of objects. Originally used for metallurgy and pharmaceuticals, assay is used to describe the analysis of anything.
The following story uses both words correctly:
At the end of the semester, Esme wrote several essays about her experiences studying geology and metallurgy in Chile.