I just had this scene in my mind in which Grace and Rocky bring the pebbles into the biodome as a field trip to show them the sea, and the forest and to explain to them how a ecosystems would usually work in earth.
A very small and curious pebble wanders off and ends getting to Grace house to then walk around it and find a garden of flowers and veggies that they want to explore. Then suddenly a human, that is not Mr.Grace, gets out of the house and not wanting to be caught the little pebble hides between the plants and decorative rocks of the garden.
The human waters the plants and takes care of the plants with care before returning to the house and opening the door. The pebble, who has not be discover yet, takes this opportunity and sneaks inside the house before the human closes the door behind him.
When Grace gets home he enters in a rush screaming about the field trip ending shortly and Rocky having to take the other students back to the classroom, because they have lost one of the pebbles and asking Simon if he could help him in finding them. He finds Simon standing over one of the chairs screaming in a high tone and swinging a frying pan at a small hyperactive pebble that is running around the chair non-stop while shrieking
At least he knows where Roach is.
Grace taking the pebble that fits in his hand from the floor: You are in a lot of troubles, buddy. I must speak with your parents.
Roach admitting defeat and just going limb in Grace's hand.
Grace showing Roach to Simon with a smile in his face: Say hi, Roach.
Roach waving one of his claws very excited.
Simon still using the frying pan to keep distance between him and Grace: But...but...
Grace patting Roach: Yes, this one a jokester I'm afraid, I'm very sorry they just sneak on me.
Simon taking a moment to think to then throw his hand to the air: Did you really named the cockroach looking pebble, Roach?!
Grace, not missing a beat: Easy to remember.