Given a cube in which you can put two massive spheres of radius 1.
What's the smallest possible value of the side - length of the cube? 
Prove that your answer is the best possible.
1. **Determine the minimum side length for one sphere:**
   - A sphere of radius 1 cm has a diameter of 2 cm.
   - Therefore, the side length of the cube must be at least 2 cm to contain one sphere.

2. **Consider the arrangement of two spheres:**
   - To fit two spheres of radius 1 cm each inside a cube, we need to consider the possible arrangements.
   - The simplest arrangement is to place the two spheres side by side along one dimension of the cube.

3. **Calculate the required side length for two spheres:**
   - If the spheres are placed side by side, the total length required along one dimension is the sum of their diameters: \(2 \text{ cm} + 2 \text{ cm} = 4 \text{ cm}\).
   - Therefore, the side length of the cube must be at least 4 cm to accommodate both spheres in this arrangement.

4. **Prove that this is the best possible solution:**
   - Any arrangement of the two spheres must fit within the cube without overlapping.
   - The minimum distance between the centers of the two spheres is 2 cm (since each sphere has a radius of 1 cm).
   - To ensure that both spheres fit within the cube without overlapping and touching the cube's sides, the side length of the cube must be at least 4 cm.
   - Any side length less than 4 cm would not allow the two spheres to fit without overlapping or extending outside the cube.

Therefore, the smallest possible value of the side length of the cube that can contain two spheres of radius 1 cm each is 4 cm.

The final answer is \(\boxed{4 \text{ cm}}\).