They know that the worms can hear them but can't see them, so they decide to use grenades as distractions to help them reach a tall building, which they ascertain that the worms couldn't reach them.
The worms can hear them. This is what they know. They can't see them. They decide to use grenades as distractions to help them reach a tall building. The worms couldn't reach them a tall building. This is what they ascertain.