Solution:

Water has a unique property: it reaches its maximum density at 4°C. This means that as water is heated or cooled from 4°C, it expands. Here’s why:

1. If the water is heated above 4°C: It begins to expand, decreasing in density. Since the beaker is already full, this expansion causes water to overflow.

2. If the water is cooled below 4°C: Water also expands when cooled below 4°C due to the formation of an open, hexagonal structure in its molecules (especially as it approaches freezing). This expansion also leads to overflow.

Therefore, the beaker will overflow if the temperature changes from 4°C in either direction: heating above 4°C or cooling below 4°C.