Father Time is an unconquerable force of nature.
As human beings, we all start out as tiny infants, but we all age into children, adolescents and then adults, until eventually, the circle of life is complete.
For some little ones, this concept is jarring.
In the case of five-year-old Sadie, who just grasped that her baby brother is not going to stay a baby forever, the realization was especially devastating:
Alex Miller uploaded this cute clip to YouTube, showing the youngster's reaction.
"Typical struggles of a 5-year-old," reads the caption of Sadie's video life lesson.
The reason children are so wonderful, though, is that this meltdown comes from a place of emotional purity, a reservoir of innocence and unconditional love.
Learning that he'll grow up someday only makes her want to love him more.
Sadie can't stop giving her brother hugs and telling him how cute his smiles are, so when he responds with a half-smile, it's enough to melt her heart and ours.
Bless all their little hearts.