Mo. Rev. Stat. § 115.443: Paper ballots, how marked — electronic voting machines, how voted.

1. Where paper ballots are used, the voter shall, before leaving the voting booth, fold his ballot so that the distinguishing marks are concealed. The voter shall place his ballot in the ballot box and leave the polling place immediately.

2. Where electronic voting machines are used, the voter shall register his vote as directed in the instructions for use of the machine and leave the polling place immediately.