Yuji Naka on Twitter:
I had bought Sonic the Hedgehog Bath Bombs at Don Quijote, tried to put one into the bathtub and took a bath with my children. The bath’s color became deep blue, and my children were delighted as it looked like Sonic. This time it is Knuckles, but I am not sure if all 5 kinds would appear. It is quite cute and has good feeling.
[Order this item on Amazon.co.jp (Japanese).]