The words kayak, kyack sound the same but have different meanings and spellings. Why do kayak, kyack sound the same even though they are completely different words?
The answer is simple: kayak, kyack are homophones of the English language.
An Inuit or Eskimo boat consisting of a light wooden frame covered with watertight skins except for a single or double opening in the center, and propelled by a double-bladed paddle.
A lightweight canoe that is similar in design.
To go, travel, or race in a kayak.
To go or travel on (a body of water) by kayak: kayaked rapids of the Colorado River.
A packsack that hangs on either side of a packsaddle.
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.