How many lips does a flower have?
Why this joke works
A Flower with Lips
Tulips sounds like two lips, which answers the question with a number and a body part while also naming flowers. The setup prepares the listener to accept a talking or humanlike blossom; the reply instead uses a familiar plant name. Keep two and lips close together when telling it aloud. A long pause between them would reveal the construction before the flower word can be heard.
A closer look
The plural makes the sound
In many accents, tulip begins more like tyoo than too, so the match is approximate rather than universal. The written answer can make the intended link clearer. This is also why the plural tulips matters: it supplies the final s needed by lips, even though the question asks about a single imaginary flower.
Fact behind the joke
Spring flowers from bulbs
Tulips grow from bulbs and flower in spring, with different cultivars blooming at different points in the season. The familiar cup-shaped flower is only one of their forms. The joke uses the plant's name, rather than a real floral structure called a pair of lips.