all of a sudden
Puberty is a (一) scary thing because one can become covered in zits all of a sudden. There are days where one just wants to stay under (下) the blanket and remain a child (子).
all at once and all of a sudden
My brother squeezed (yī) his zits (zǐ ) in an attempt to get rid of them and shut (xià) up the people making fun of him.
yi1 xia4 zi3 and yi xia zi