In dehydration synthesis, the water molecule comes from the reactants themselves.

Here's how it works:

1. Two monomers (smaller molecules) come together to form a polymer (larger molecule).

2. The hydroxyl group (-OH) from one monomer and a hydrogen atom (-H) from the other monomer are removed.

3. These removed atoms combine to form a water molecule (H2O).

Essentially, the water is a byproduct of the reaction. It's not added from an external source; it's created from the molecules that are joining together.

Think of it like building a chain with links. Each link represents a monomer. To join two links, you remove a small part of each link and use those parts to create a "connection" (the water molecule).