The kingdom of heaven is like a seed. (Matthew 13:31) Planted and properly cultivated, this seed will grow into a magnificent tree, for birds to nest in and people to rest under. But some, seeing or hearing of the majestic trees of others, decide to shortcut the process and put fake, hollow trees up over their growing seeds. They ornament these "trees" with finely crafted leaves and blossoms that look convincing unless you get right up close. Whole forests of these hollow trees pop up and people give them names and pay them great respect. But they are nothing but shells. Meanwhile the real seeds are stuck inside the shells. Without sunlight and fresh air, they can't grow and just stagnate or even wither.
Other people without seeds also desire the grandeur of the fully-grown trees. No problem! They get the synthetic trees all the same and put them up over nothing. No one can tell the difference. Almost no one, anyway. Still others don't see the point of the trees in the first place and go without, or prefer nice bushes or ivy.
How do you tell shells from real trees? How can you tell if a shell has a seed inside it? What is your shell made of?