That's correct. Microwave ovens work by emitting electromagnetic waves, specifically microwaves, which have a particular frequency that interacts strongly with water molecules. These microwaves cause the water molecules to vibrate rapidly, generating heat in the process. This heat then spreads to the surrounding food, heating it throughout.
Since wood is not composed primarily of water molecules, it does not strongly interact with microwaves in the same way. As a result, the wood will not heat up significantly in a microwave oven. However, it's worth noting that microwaves can still generate some heat in other materials, including wood, but the effect is generally much less pronounced compared to water. The heating may occur indirectly through the transfer of heat from the heated food or through secondary effects, such as absorption and conduction.
Nevertheless, it is not recommended to put objects in a microwave oven that are not explicitly designed for microwave use, as it may cause damage to the oven or the object itself. Microwave ovens are specifically designed to heat food efficiently and safely, taking into account the interaction of microwaves with water molecules.