The concept of upper and lower case letters can be kind of confusing.
"I thought this was B . . . "
"well . . .and so is this b, but not this d."
Up to this point in a child's life, they have been learning very concrete concepts like a shoe is a shoe regardless of where it is or what direction it faces. Even a triangle is still a triangle no matter which way it goes.
Now letters come into play . . . They have to grasp that a B can have two totally different looks, but is no longer a b if it flips to look like this d . . . and that is confusing!
I found this fun elvis-style video from Between the Lions that would be fun to use when helping a child understand this concept.
Have fun. . . you might just find yourself singing along!