Time point T0: Cat is holding a nerf gun, you are holding a nerf gun.
Time point T1: Cat is holding two nerf guns.
That is a fact.
dT = T1-T0
Time point T0: Cat is holding a nerf gun, you are holding a nerf gun.
Time point T1: Cat is holding two nerf guns.
That is a fact.
dT = T1-T0