Why do Meg and Calvin have so much trouble explaining Mrs Whatsit to the creature on Ixchel?

Meg can't use language, she must communicate with Aunt Beast by thought.

Meg and Calvin have lots of trouble explaining Mrs. Whatsit to the beasts because every description she tries to use is just a physical description. Since the beasts don't have or need the ability to see, Meg’s descriptions mean nothing to them. The beasts ask her to just think about Mrs. Whatsit, to think about her personality and how she behaves around others, but Meg can't get past the way she looks and describe her character to the beasts.