why does mrs. Murray get pale when mrs. Whatsit is talking? What is she talking about

Mrs. Murray's face goes white when Mrs. Whatsit talks about the tesseract.

“Wet socks don’t bother me. I just didn’t like the water squishing around in my boots. Now don’t worry about me, lamb.” (Lamb was not a word one would ordinarily think of calling Mrs. Murry.) “I shall just sit down for a moment and pop on my boots and then I’ll be on my way. Speaking of ways, pet, by the way, there is such a thing as a tesseract.”
Mrs. Murry went very white and with one hand reached