What is the cause of geometrical isomerism in alkenes ?

Explanation

### Step-by-Step Solution

1. **Definition of Geometrical Isomerism**:
Geometrical isomerism occurs when compounds have the same molecular formula but differ in the spatial arrangement of atoms. This means that certain parts of the molecule are fixed in position relative to each other, preventing them from being interconverted by simple rotation or flipping.

2. **Example of Geometrical Isomerism**:
Consider a compound with two identical groups (e.g., CH3) and two hydrogen atoms attached to a double bond. The arrangement of these groups can lead to different isomers (cis and trans forms). For instance, in one isomer, the two CH3 groups may be on the same side (cis), while in another isomer, they may be on opposite sides (trans). These two forms cannot be converted into each other by mere rotation.

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