A black cat named Macak was Tesla's favorite childhood companion. Decades later, in a 1939 letter to a 12-year-old, Tesla recalled how this cat shocked him into a lifelong fascination with electricity. Part-Time Genius co-host Will Pearson read some of the letter on an episode about cats. "In the dusk of the evening as I stroked Macak's back, I saw a miracle which made me speechless with amazement.

Macak's back was a sheet of light, and my hand produced a shower of crackling sparks loud enough to be heard all over the house ... My mother seemed charmed — 'Stop playing with the cat,' she said. 'He might start a fire.' But I was thinking abstractly. Is Nature a gigantic cat? If so, who strokes its back?... I cannot exaggerate the effect of this marvelous night on my childish imagination. Day after day I have asked myself, what is electricity?"