If they are thick, they have a hard time to rise
Time
Because dogs can't whistle. (X-post from r/dadjokes)
He was having a hard time looking up.
They both have a hard time pulling off a twist.
A rose by any other name would smell as sweet. But, if roses were called "herpes" they'd make terrible Valentine's Day gifts.
Tulips on your organ EDIT: This blew up. (No pun intended)
They both started out black and blue then became white and golden
30 of them are triggered every second
I make my own dough". My sister Katie came up with this one.
You're my life's devotion. I knead you!