The heating element, under the immovable left lower blade. Traditionally, the material used to make the ends of the film adhere together has been called cement. It is never called glue or paste. Both of these words are used in the trade to describe poor or amateurish editing. The cement must always be kept fresh.
Fresh cement is always quite thin, never thick or gooey.
If you have doubts about the freshness of your cement, dump it out and refill the bottle. If cement is spilled on film, furniture or clothing, it can cause serious damage.