In general, substances that contain a high amount of water molecules tend to heat up the fastest in a microwave. This is because microwaves work by emitting electromagnetic waves that excite the water molecules, causing them to vibrate and generate heat.
Therefore, items such as liquids, moist foods, and foods with high water content, like fresh vegetables or fruits, typically heat up quickly in a microwave. These substances absorb the microwave energy efficiently and convert it into heat rapidly.
On the other hand, substances with low water content or with insulating properties, such as dry foods or certain types of plastics, may not heat up as quickly or evenly in a microwave. The heat transfer in these materials primarily occurs through conduction from the hot exterior to the interior, which can be slower compared to the direct absorption of microwaves by water molecules.
It's important to note that when using a microwave, you should always follow the manufacturer's instructions and exercise caution to prevent overheating or uneven heating of food, as different microwaves may have varying power levels and distribution of microwaves.