He knows how!
Yes a shower shower.
This is how I do it:
Shower on luke warm, not hot, not cold, not warm warm.
Point le shower head at wall.
Put le chameleon on plant.
Put said plant containing said chameleon in shower
to where half the plant is getting hit by the droplets
bouncing off the wall.
The other half will be for escaping the "rain" if nessecary.
Leave for 30min.
This gives the chameleon plenty of time to decide that it is indeed raining, and that it is indeed time to drink. You should go ahead and fill out the how to ask for help form maybe, just to make sure everythings ok.