How can fire be contained inside hail?
Rashi: It was a miracle within a miracle 1 - two opposing forces made peace in order to fulfill the will of their Creator.
Rashbam: Refer to 9:14:1:3.
Seforno: It wasn't. What happened was that the power of the swiftly moving hailstones heated up the air, turning it into fire 2 and causing thunder.
What does "Mislakachas b'Soch ha'Barad" mean?
Hadar Zekenim (citing the Pesikta): It means that they formed a sort of mixture.
Targum Yonasan: It means that the fire jumped about inside the hail.
Chizkuni: It means that after the hail struck someone, the fire took him (Lokcho) and burned him.
Ba'al ha'Turim: It means that sparks and light came out. 1
Ba'al ha'Turim: As in the verse, "v'Eish Mislakachas, v'Nogah Lo Saviv" (Yechezkel 1:4).