Title text: Dad, where is Grandpa right now?
Lego blocks are a popular building toy, which Cueball here uses to describe a philosophical conundrum: the distinction between a composition, and the collection of parts that make up that composition. For example, the Lego blocks he and his daughter (Ponytail) used to make a house are still around; they were put back into the bin, and can be used on future designs. However, the house itself, as a specific combination of those blocks, is gone. It ceased to exist when they took it apart. In essence, they "killed" the house.
Later in her life, Ponytail extends this thinking to humans and organ donation. The US has an opt-in system for organ donation; in the event that you die, any of your organs or tissues that remain functional after your death can be donated for transplantation or medical research, provided you've opted into the organ donor registry. Ponytail compares her organs to the Lego blocks she's carrying - even if she (the composition) dies, her organs (the pieces) can continue to serve another. As such, she is compelled to register as an organ donor.
The title-text is the same question asked in the first panel, asked from this new perspective - instead of asking where the Lego house went, the questioner (presumably a young child, possibly still Ponytail) is asking where his/her Grandpa went. Humans are a composition of many parts; the parts are (usually) buried or cremated when we die, but the composition is something else entirely. (What exactly happens to a human composition after death is a question for religious debate, but we know for sure it doesn't stay here.)
- [Ponytail and her father Cueball are putting away Lego bricks.]
- Cueball: When you take apart a Lego house and mix the pieces into the bin, where does the house go?
- Ponytail: It's in the bin.
- Cueball: No, those are just pieces. They could become spaceships or trains. The house was an arrangement. The arrangement doesn't stay with the pieces and it doesn't go anywhere else. It's just gone.
- [Ponytail, older, is standing at a desk. She's holding a couple of Lego bricks.]
- [She looks at the bricks.]
- [She checks off a box next to the words "Organ Donor" on something on the desk.]