Butter is difficult to wash off your hands with just water because it is a lipid that is nonpolar and hydrophobic. This means that butter does not mix well with water, which is polar. Since water molecules are polar, they are attracted to other polar substances and can easily dissolve them. However, nonpolar substances like butter do not interact well with water, making it hard to wash off with water alone.
To effectively remove butter from your hands, you would need to use soap. Soap molecules have both polar and nonpolar ends. The nonpolar end of the soap molecule attaches to the butter, while the polar end attaches to the water, allowing the butter to be washed away.