a scientist turns his car into a spaceship and with his kid friend they go with a e.t. turtle with one arm

[hialoidea]

its al old sci fi classic, where the scientist discovers that space travel is actually dimensional travel, and we find what we are, so this turtles with one arm are pretty nice and chill and help each other, but other creatures aren liek that, space travel is actually interdimensional travel, it´s pretty old and classic and i cant find it