Crum is the alcohol mixture of cream soda^ and rum*. One shot of rum mixed with a bottle of cream soda will make the rum almost tasteless. Typically a 1/3 glass of rum and the rest cream soda will make for an enjoyable harder mixture know as a Pro-V mixture. Ice is not necessarily needed but is preferred.
* Clear brands of Rum are typically used (BACARDI®)
^ IBC® cream soda is the preferred brand.
I needed a quick drunkenness, after mixing a Crum on the rocks my problems melted away.