Your mom's body will absorb and fill up with any liquid it enters in contact with. Your mom has light yellow skin and is spotted with light olive-green dots, just like SpongeBob. It was originally a normal yellow cake, because of this, you thought it was poisonous to eat. But it's the complete opposite. This is what gave your mom the color and the abilities it comes with for her.

A few weeks ago at the end of school year, it was the end of the school year and it was summer, and you celebrated your mom's birthday with the sponge cake that you and your mom made for her, but it didn't affect you, it only affected your mom's physiology which turned her into a growable/absorbing/absorptive human sponge hybrid with a wide body. Weeks later, your mom has gotten to the drastic proportion changes from her slim body and is living happily with herself and you.

Mom: Hey sweetheart! How was your last day of school today, did you make any new friends today before summer began?