This makes the heart grow "FOUNDER"... Not FONDER. What is it?

Answer: The letter U.

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This thing all things devours: Birds, beasts, trees, flowers; Gnaws iron, bites steel; Grinds hard stones to meal; Slays king, ruins town, And beats high mountain down. What am I?
Three doctors said that Robert was their brother. Robert said he had no brothers. Who is lying?
Three lives have I. Gentle enough to soothe the skin, Light enough to caress the sky, Hard enough to crack rocks. What am I?
Three matches are sitting on a table. Without adding another make for three matches four. You are not allowed to break any of the matches. How can this be done?
Three men are walking across a green and luscious field. Only two of them are wearing rubber boots and yet the feet of the third man remain dry. Why?
Three pirates, One Eye, Long John, and Peg Leg, were gambling with pieces of gold. All the winnings were piled up on the table. One Eye said, 'I have won 1/2' and took a large handful or two. Long John said, 'I have won 1/3' and took a handful. Peg Leg said, ' I have won 1/6' and took a small handful. One Eye said, ' I have taken too much' and returned a half. Long John said, 'I have taken too much' and returned a third. Peg Leg said, 'I have taken too much and returned a sixth. The money on the table was then shared out equally and they had 42 pieces each. One Eye said, 'I have now 1/2 of the total originally.' Long John said, ' I now have 1/3 of the originally.' Peg Leg said, 'I have now 1/6 of the originally.' How much was on the table originally?
Three playing cards in a row. Can you name them with these clues? There is a two to the right of a king. A diamond will be found to the left of a spade. An ace is to the left of a heart. A heart is to the left of a spade. Now, identify all three cards.
Three working women have different careers. If only one of statements 1, 2 and 3 are true, can you tell whether or not Mary is a nurse? 1. This statement is only true if statement 5 is false. 2. This statement is true if statements 4 or 5, or both 4 and 5 are true. 3. This statement is false only if both statements 6 and 1 are true. 4. Mary is a nurse 5. Karen is an artist. 6. Sarah is a photographer.
Tom and his younger sister were fighting. Their mother was tired of the fighting and decided to punish them by making them stand on the same piece of newspaper in such a way that they couldn't touch each other. How did she accomplish this?
Tomorrow Is Neither Wednesday Nor Thursday. Yesterday Was Not Friday Or Saturday. Today Is Not Thursday Nor Monday Nor Sunday. What Day Is Today?
Tool of thief, toy of queen. Always used to be unseen. Sign of joy, sign of sorrow. Giving all likeness borrowed
Trains travel from one town to another town all day, always on the same track, always going nonstop and at the same speed. The noon train took 80 minutes to complete the trip, but the 4 PM train took an hour and 20 minutes. Why?