The day of Pentecost

“All of them were filled with the Holy Spirit and began to speak in other tongues as the Spirit enabled them.” Acts 2:4
In Acts 2 we see the beginning of the Church Dispensation: the pouring out of the Holy Spirit upon believers, as prophesied by Joel. The particular manifestation was the speaking in other tongues (or languages). People heard the disciples speaking in languages from fifteen different regions (see Acts 2:9-11). Some marvelled, while others mocked (see Acts 2:12-13). We can learn a lot about what speaking in tongues is, by making some observations about this first instance of it.
Firstly, the speaking was controllable by the speakers. It was not out-of-control babbling. The speaker had the ability to turn it on and off (see 1 Cor. 14:32). Later, Paul would write to the Corinthians about this gift of the Spirit, pointing out the need for tongues to be used in an orderly fashion, and even in some situations for the speaker to remain silent (i.e. when there is no interpreter, 1 Cor. 14:27-28).
Secondly, the tongues were understandable by various people in the crowd (Acts 2:8). It would seem from the Scriptures that there are two kinds of tongues: one which is a ‘heavenly language’, by which our spirit can talk to God (1 Cor. 14:2); another which is an actual language on earth, which the speaker does not know and has not learned.
Thirdly, the interpretation of the tongues being spoken was that the speakers were praising God. Nowhere in Scripture do we see ‘messages in tongues’ being given, where a tongue is spoken and the ‘interpretation’ is a message for those hearing it. (Such incidents are more likely the gift of prophecy, or a word of knowledge, rather than interpretation of the tongue.)

*For some other posts on the gift of tongues, see here, here, and here.