Easiest way to explain that - The baby is pulling its tail. (no apostrophe)
Is there any point to this strange post?
why not it's.
It would have been clearer if you had written 'What is the easiest way of explaining why "its" has no apostrophe in The baby is drinking its milk?" The fact that babies do not have tails did not help.
It's is the contracted form of it has or it is.
There are no apostrophes in possessives: my, mine, your, yours, his, her, hers, its, our, ours, their, theirs.