The words cereal, serial sound the same but have different meanings and spellings. Why do cereal, serial sound the same even though they are completely different words?
The answer is simple: cereal, serial are homophones of the English language.
A grass such as wheat, oats, or corn, the starchy grains of which are used as food.
The grain of such a grass.
Any of several other plants or their edible seed or fruit, such as buckwheat or grain amaranth.
A food prepared from any of these plants, especially a breakfast food made from commercially processed grain.
Of, forming, or arranged in a series.
Published or produced in installments, as a novel or television drama.
Relating to such publication or production.
Responsible for a series of usually criminal acts over a period of time: a serial arsonist.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.
If they are spelled the same then they are also homographs (and homonyms); if they are spelled differently then they are also heterographs (literally "different writing").