SB 8.11.39

tāṁ daivīṁ giram ākarṇya
 maghavān susamāhitaḥ
dhyāyan phenam athāpaśyad
 upāyam ubhayātmakam
Word for word: 
tām — that; daivīm — ominous; giram — voice; ākarṇya — after hearing; maghavān — Lord Indra; su-samāhitaḥ — becoming very careful; dhyāyan — meditating; phenam — appearance of foam; atha — thereafter; apaśyat — he saw; upāyam — the means; ubhaya-ātmakam — simultaneously dry and moist.
Translation: 
After hearing the ominous voice, Indra, with great attention, began to meditate on how to kill the demon. He then saw that foam would be the means, for it is neither moist nor dry.