Everything about his grandma's house looks a little different now that our narrator is a mouse, but Grandmamma rigs up some cool contraptions to let him open doors and turn on lights and all that jazz.
The only bad part (to Shmoop, at least) is that, as a mouse, he doesn't like sweets and chocolate anymore. That kind of stinks.
One night, he's talking to his grandma, wondering what might have happened to Bruno, and he thinks of a question: "How long does a mouse live?" (21.14).
It turns out Grandmamma had done some research into that, and mice-people live three times longer than regular mice. But that's still only about nine years.
You might think that's bad news, but our narrator is thrilled. He doesn't want to live longer than his grandma and they both figure she won't be around much longer than nine more years anyway.
Then Grandmamma tells our narrator something else quite interesting: a mouse's heart beats five hundred times a minute. (That's actually true. For comparison, a human's heart beats about seventy times a minute – quite a difference.)
She tells him that she can hear his heart beat when they're cuddled up together, and that it sounds like a little humming sound.
They sit in silence for a while, enjoying each other's company, until Grandmamma asks him if he's sure he doesn't mind being a mouse.
He responds: "It doesn't matter who you are or what you look like so long as somebody loves you" (21.52). Cheesy alert, but, also, awesome alert!