Snapdragons (Antirrhinum majus) are hours (well minutes) of fun. Hold the flower by the sides of the tube and squeeze:
There's some interesting botany behind this. Flowers can be well-adapted for particular pollinators. But often that doesn't stop other visitors entering the flower and stealing nectar or pollen without effectively pollinating it. Some flowers have a palate that closes the corolla throat so that only a pretty strong insect can force its way inside. Snapdragons have this strategy, as do some of their relatives, like ivy-leaved toadflax and Linaria.