Why does the narrator answer "Go and play with Bhola, Mini! I am busy!" with "a grave face"?

My five years' old daughter Mini cannot live without chattering. I really believe that in all her life she has not wasted a minute in silence. Her mother is often vexed at this, and would stop her prattle, but I would not. To see Mini quiet is unnatural, and I cannot bear it long. And so my own talk with her is always lively.

One morning, for instance, when I was in the midst of the seventeenth chapter of my new novel, my little Mini stole into the room, and putting her hand into mine, said: "Father! Ramdayal the door-keeper calls a crow a krow! He doesn't know anything, does he?"

Before I could explain to her the differences of language in this world, she was embarked on the full tide of another subject. "What do you think, Father? Bhola says there is an elephant in the clouds, blowing water out of his trunk, and that is why it rains!"

And then, darting off anew, while I sat still making ready some reply to this last saying, "Father! what relation is Mother to you?"

"My dear little sister in the law!" I murmured involuntarily to myself, but with a grave face contrived to answer: "Go and play with Bhola, Mini! I am busy!"

The window of my room overlooks the road. The child had seated herself at my feet near my table, and was playing softly, drumming on her knees. All of a sudden Mini left her play, and ran to the window, crying, "A Cabuliwallah! a Cabuliwallah!" Sure enough in the street below was a Cabuliwallah, passing slowly along.