You are probably thinking that there is a misprint in the previous sentence, but tulips indeed have tepals and not petals. For an explanation we will need to have a closer look into botanics, more exactly into its part which studies the flower structure.

Let us see! Usually the flower is composed of a whorl of colourful petals, lined on the outside by a whorl of tiny green leaflike scales called sepals. The tulip, as you may notice, seems to be deprived of any sepals. However, out of the six petals one would think the tulip has, only the three inner ones are strictly speaking petals, whereas the three outer ones are sepals that have modified to imitate the form and chromatic pattern of the petals and are formally holding the place of the missing sepals.

In botanical terms, within such a flower structure, the elements that would normally be called petals and sepals are called with the joint term tepals. That is how the tulip got to have tepals instead of petals.