Here's why:
* Gendered Adjectives: In Spanish, adjectives often have different forms depending on the gender of the noun they modify.
* Cansado/Cansada: "Cansado" is the masculine form of the adjective meaning "tired," while "cansada" is the feminine form.
* Estoy: The verb "estoy" is the first person singular present tense conjugation of the verb "estar" (to be). It's used for both men and women.
Therefore, "estoy cansada" uses the feminine adjective "cansada," indicating the speaker is female.
To express the same idea for a man, you would use "estoy cansado."