real
Explanation
The Joke
A child asks a parent, "What's the matter, Mom?" The parent (or another adult) says, "Is the tooth fairy real?" A second adult responds, "If you love her in your heart, then she's as real as anything." But a third person (or the same person continuing) delivers the punchline: "But you love Batman, and Batman isn't real." The child's innocent question about the tooth fairy gets turned into an inadvertent philosophical trap. The well-meaning adult tries to give the comforting answer that belief makes things real, but the child (or another person) immediately finds the logical flaw: you can love fictional characters too, so love alone can't be the criterion for something being real.
The Humor
The comic skewers the common parental dodge of using vague emotional language ("if you love her in your heart, she's real") to avoid directly answering a child's question about whether something exists. The humor comes from how quickly and easily this sentimental logic collapses under the slightest scrutiny. The Batman counterexample is perfect because it's something both children and adults clearly love but equally clearly understand to be fictional. It highlights how adults often give children non-answers that don't hold up to even basic logical examination, and how children can sometimes see through these evasions with devastating clarity.