To answer this question, you need to look at the geometries of the molecules CO2 and H2O. In order to find out bond angles in a molecule, you can use a method called VSEPR theory (valence shell electron pair repulsion) in order to simulate what a certain molecule would look like. This theory relies on two factors: how many bonded pairs of electrons (BPs) and lone pairs of electrons (LPs) there are around a central atom. (By bonded pairs, I mean pairs of electrons that are shared between two atoms in a covalent bond.) Usually, the central atom of a molecule is in the front of the name: in CO2, it would be C. In methane (CH4), the central atom, carbon, is bonded to four hydrogen molecules. Each carbon-hydrogen bond represents a covalently bonded pair of electrons, so you would have 4 BPs. On the central carbon atom there are no electrons that aren't being shared in a bond (lone pairs), so you would have no LPs.

Depending on the amount of bonded pairs or lone pairs that you have, you can have different ways the molecules are set up. This is because all pairs of electrons, regardless of whether they are bonded pairs or lone pairs, want to stay as far away from each other as possible. A great tool to simulate all the different possible geometries in a molecule is this online interactive simulation: https://phet.colorado.edu/sims/html/molecule-shapes/latest/molecule-shapes_en.html. Something to keep in mind is that a double or a triple bond counts as only one bonded pair, because it shares the same purpose as all other covalently bonded pairs of electrons: sharing electrons between two molecules.

        In carbon dioxide:

Carbon has four valence electrons; therefore, it needs four more electrons to satisfy its valence shell. Oxygen has six valence electrons; therefore, it needs two more electrons to satisfy its valence shell. Carbon can form double bonds with two oxygen molecules in order for all of their valence shells to be satisfied.

Carbon dioxide (CO2): O=C=O

Here, the central atom (carbon) has no electrons that are not bonded to other molecules, so 0 lone pairs. The carbon atom also has two double bonds with each oxygen, so 2 bonded pairs. This adds up to a total of 2 pairs of electrons. The two pairs want to get as far away from each other as possible, so they line up on opposite sides of the molecule and form a linear structure. Because of this, the bond angles turn out to be 180 degrees.

        In water:

Hydrogen has only one valence electron. Hydrogen only needs two electrons to satisfy its valence shell, so it needs one more electron. Oxygen has six valence electrons; therefore, it needs two more electrons to satisfy its valence shell. Oxygen can form covalent bonds with two hydrogen atoms in order for all of their valence shells to be satisfied.

Water (H2O): H--O--H

Here, the central atom (oxygen) forms two single bonds with two hydrogen molecules, so it has two bonded pairs. However, the central oxygen atom also has two lone pairs of electrons left that were left over and were not bonded to anything. So, water has 2 bonded pairs and 2 lone pairs of electrons, making 4 total pairs of electrons. All of those electron pairs, regardless of whether they are bonded or lone pairs, will repel each other as much as possible. Because there are 4 total pairs of electrons instead of 2, the molecule forms a tighter tetrahedral structure and therefore will have a different molecular shape than carbon dioxide. Because of this, the bond angles turn out to be 104.5 degrees.