On their third day at the museum, the children wash their clothes at the laundromat, and then, on Claudia's insistence, they go to the New York Public Library to read about Michelangelo. Claudia begins her research confident that she can become an expert in one morning, but is quickly daunted by the vast amount of material on the artist. Jamie learns that many of Michelangelo's works were lost, but the children find no sign of Angel.
Finally, realizing that the experts probably know more than they do, the children depart. Jamie finds a candy bar on the sidewalk, and, despite Claudia's warnings that it is probably filled with dope, he eats it.
The children get a large lunch and return to the museum. They hide on the toilet seats as usual, and Jamie hears two workers come in long after the bell rings. The workers are discussing moving Angel, which is why they have stayed long after the close of the museum. Claudia and Jamie avoid detection again, and sort their laundry.
They also decide to take a bath, and do so at the fountain by the restaurant in the museum. They discover that many people have thrown coins into the fountain - the two children now have a source of income.
Afterward, the children visit Angel once again before heading to bed. They discuss their lack of homesickness before going to sleep.
It is now Sunday morning. Because the museum opens late this day, children go to the medieval art section and say some prayers.
Then they go to look at the statue of Angel again. Claudia wishes she could hug it, which Jamie teases her for. Suddenly, a guard appears, and the children have to hide quickly.
They find themselves by the pedestal that Angel had stood on the day before, and they notice a strange crumple in the fabric. A symbol has been impressed on the soft fabric: three overlapping circles with an ‘M’ in the center. Jamie recognizes this symbol from the books they read the day before. The children are determined to find the book again and figure out what it means. For Claudia, Angel is a symbol for her desire to be different, and a mystery that she feels she needs to solve before she can return home.
When the museum opens, the children go to the museum bookstore and find the strange symbol impressed onto the fabric - it is the artist's mark of Michelangelo, and it was on the bottom of the statue of Angel. This is strong proof that Michelangelo indeed carved the statue.
The children get brunch and ponder how they can pass on their discovery without revealing that they have been living in the museum. Claudia is determined that they will not go home until they have discovered the truth.
They decide to write an anonymous letter to the director of the museum. They use the typewriter in from of Olivetti's to do this - the typewriter there is intended for customers to get used to the feel of the machine so that they will buy one inside. They type a letter, and list their address as an anonymous P.O. Box. Then they tour the Ancient Greek art section before hiding in the bathrooms for close again.
The children know that it might be impossible for them to know more than the experts about the statue, but they reason that perhaps they can use their extended proximity to the statue to determine its secrets. Indeed, it is because they live with the statue that they make the exciting discovery of the mark imprinted on the fabric.
But what could this mark mean? Claudia and Jamie assume that no one else has noticed it because it is on the bottom of the statue. They believe that they are the only ones who know about this important piece of information. Logically, the symbol carved into the base of the statue - Michelangelo's artist's mark - is no guarantee that the statue was carved by the sculptor himself. This widely known symbol may have been carved into the base by an imposter attempting to create a believable fake.
As they bathe in the fountain, the children make another exciting discovery - the fountain has been used as a wishing well! They happily scoop up the coins from the bottom. Jamie comments, “Someone very rich must have tossed in this quarter,” to which Claudia replies “Someone very poor. Rich people have only penny wishes” (pg. 84). Claudia reasons that only poor people could face such troubles that they would be willing to sacrifice a whole quarter to solve them. Rich people are shielded from such difficulties, so they would only throw in a penny for a wish. This quotation is a great example of the maturity and precociousness exhibited by the two Kincaid children, who are often more intelligent and observant than one would guess from their age.
The children also bond about their peculiar lack of homesickness. Jamie suggests that homesickness, like thumb-sucking, is what happens when you're not very sure of yourself. This simile implies that thumb-sucking, like homesickness, is only for young children. It also suggests that Claudia and Jamie are unusually sure of themselves.