CHICAGO (CBS) — When Shari Hart was diagnosed with leukemia last week, her 14-year-old son wanted to do something special.

So, this past weekend he wrote her a simple message: “HI MOM,” with a smiley face inside the O.

Two simple words that had a big impact, because of the effort it took. Her son, Will, didn’t simply write those words in a text message or on a greeting card, he wrote it in giant letters in the snow on the roof of the parking garage across the street from his mom’s room at Rush University Medical Center.