For me it is the fact that our blood contains iron. I earlier used to believe the word stood for some ‘organic element’ since I couldn’t accept we had metal flowing through our supposed carbon-based bodies, till I realized that is where the taste and smell of blood comes from.
Short: It completes a full 360° of the sun before the planet itself does a full 360° spin.
A few sentences longer:
In planet Earth human terms, we have defined one day as “how long it takes the planet to do a full 360 degree rotation”. Example: You spin a basketball on your finger and it does one full rotation.
A year to us is “how long it takes the planet to go around the sun”. Example: You hold a basketball out in front of you and you do one full rotation.
Now, to confuse people further, read about the difference between a solar day and a sidereal day.