Mark 8:14-21

<a href="http://bible.cc/mark/8-14.htm" >14</a>Now the disciples had forgotten to take bread, neither had they in the ship with them more than one loaf. <a href="http://bible.cc/mark/8-15.htm" >15</a>And he charged them, saying, Take heed, beware of the leaven of the Pharisees, and of the leaven of Herod. <a href="http://bible.cc/mark/8-16.htm" >16</a>And they reasoned among themselves, saying, It is because we have no bread.<a href="http://bible.cc/mark/8-17.htm" >17</a>And when Jesus knew it, he saith unto them, Why reason ye, because ye have no bread? perceive ye not yet, neither understand? have ye your heart yet hardened? <a href="http://bible.cc/mark/8-18.htm" >18</a>Having eyes, see ye not? and having ears, hear ye not? and do ye not remember? <a href="http://bible.cc/mark/8-19.htm" >19</a>When I brake the five loaves among five thousand, how many baskets full of fragments took ye up? They say unto him, Twelve. <a href="http://bible.cc/mark/8-20.htm" >20</a>And when the seven among four thousand, how many baskets full of fragments took ye up? And they said, Seven. <a href="http://bible.cc/mark/8-21.htm" >21</a>And he said unto them, How is it that ye do not understand?

What is the tie between the leaven warning and the reference to the feeding miracles?