system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does a twin "absorb" the other twin in the womb?
gpt
In the very early phases of the developing embryo like the blastocyst it's just a ball of undifferentiated cells.  There hasn't been any activity yet to distinguish the individual components: what's going to be the front or the back?  Which end is the head and which end is the butt?

Because there's no distinction between individual parts or the destinies of individual cells yet, the embryo has a very high ability to tolerate gains/losses of cells.  This can lead to the absorption of another embryo where the cells of both lines intermingle more or less evenly with each other.  This leads to what's called a "chimeric" organism.