THE ARRIVAL OF SKULLGREYMON
(Episode 16)

Izzy figures out and explains to the other children that the Digimon need two things to digivolve:

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their human partner must be in danger, and 

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they must have a lot of energy (which they get by eating -- lots!). 

Since Tai is the only one with a crest so far, he gives Agumon a lot of food.  That way Agumon will be able to digivolve to his Ultimate form if the need arises.

Joe's tag leads the group to a Coliseum where they hope to find his crest, but Etemon sends an evil Greymon to attack them.  For awhile it's Greymon versus Greymon.  But then Tai gets the bright idea to put himself in danger, ON PURPOSE, to get Greymon to evolve.  Not a wise thing to do -- Agumon digivolves to a new form, SkullGreymon -- a really nasty digimon!  

SkullGreymon defeats evil Greymon, then turns on his friends before he runs out of energy and digivolves back to Koromon.  Thank goodness!

After all the excitement Joe finally finds the crest of reliability.