Here's why:

* Exothermic reactions release heat into the surroundings. The heat released causes the temperature of the beaker and its contents to increase, making it hot to the touch.

* Endothermic reactions absorb heat from the surroundings. This would cause the beaker to feel colder.

Examples of exothermic reactions:

* Burning fuel

* Mixing acids and bases

* The reaction of sodium with water

Important Note: Never perform experiments with unknown substances without proper safety precautions and supervision.